Steam segfaults when usb modules are not loaded.

I realize most people have usb stuff enabled and this is kind of niche, but I figured I should report this
- for those of us who prefer leaving usb disabled (such as myself) and
- so this doesn't bite people later since it could have been a pain to debug.

$ lsmod | grep hcd
$ steam
Running Steam on bedrock 1.0alpha3 64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
unlinked 0 orphaned pipes
removing stale semaphore last operated on by process 8469 with name 0eBlobRegistryMutex_4FF9F151DC37CD9FA43F58C5698ABBFA
removing stale semaphore last operated on by process 8469 with name 0eBlobRegistrySignal_4FF9F151DC37CD9FA43F58C5698ABBFA
removing stale semaphore last operated on by process 8469 with name 0emSteamEngineInstance
removing stale semaphore last operated on by process 8469 with name 0eSteamEngineLock
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
[0713/115819:ERROR:bus.cc(261)] Failed to connect to the bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory
[0713/115819:ERROR:bus.cc(261)] Failed to connect to the bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory
[0713/115819:WARNING:proxy_service.cc(958)] PAC support disabled because there is no system implementation
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
Installing breakpad exception handler for appid(steam)/version(1373418487_client)

** (steam:8635): WARNING **: Couldn't connect to system bus: Failed to connect to socket /var/run/dbus/system_bus_socket: No such file or directory
PulseAudio connect failed (used only for Mic Volume Control) with error: Access denied
Installing breakpad exception handler for appid(steam)/version(1373418487_client)
Generating new string page texture 7: 128x256, total string texture memory is 131.07 KB
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/assert_20130713115808_1.dmp
/home/paradigm/.local/share/Steam/steam.sh: line 704:  8635 Segmentation fault      $STEAM_DEBUGGER "$STEAMROOT/$PLATFORM/$STEAMEXE" "$@"
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-6a26d6d5-1bfd-4166-a9ab-1a5e32130713
$ sudo modprobe usb_common usbcore ehci_hcd uhci_hcd xhci_hcd usb_storage usbmouse
$ steam
<steam starts normally>

I figured this out via DEBUGGER=gdb steam and seeing 'usb' in the backtrace:

(gdb) bt
#0 0xf7c4045f in ?? () from /lib/i386-linux-gnu/libc.so.6
#1 0xf7c404dd in opendir () from /lib/i386-linux-gnu/libc.so.6
#2 0xf150ec2e in ?? () from /home/paradigm/.local/share/Steam/ubuntu12_32/steam-runtime/i386/lib/i386-linux-gnu/libusb-1.0.so.0
#3 0xf150704c in libusb_get_device_list () from /home/paradigm/.local/share/Steam/ubuntu12_32/steam-runtime/i386/lib/i386-linux-gnu/libusb-1.0.so.0
#4 0xea7066ab in ?? () from /home/paradigm/.local/share/Steam/ubuntu12_32/steamclient.so
#5 0xea816206 in ?? () from /home/paradigm/.local/share/Steam/ubuntu12_32/steamclient.so
#6 0xea8176b0 in ?? () from /home/paradigm/.local/share/Steam/ubuntu12_32/steamclient.so
#7 0xf1ec349a in SteamThreadTools::CThread::ThreadExceptionWrapper(void*) () from /home/paradigm/.local/share/Steam/ubuntu12_32/libtier0_s.so
#8 0xf1ec15eb in ?? () from /home/paradigm/.local/share/Steam/ubuntu12_32/libtier0_s.so
#9 0xf1ec1f1d in CatchAndWriteMiniDumpExForVoidPtrFn () from /home/paradigm/.local/share/Steam/ubuntu12_32/libtier0_s.so
[#10](/issue/ValveSoftware/steam-for-linux/10) 0xf1ec1f6f in CatchAndWriteMiniDumpForVoidPtrFn () from /home/paradigm/.local/share/Steam/ubuntu12_32/libtier0_s.so
[#11](/issue/ValveSoftware/steam-for-linux/11) 0xf1ec511b in SteamThreadTools::CThread::ThreadProc(void*) () from /home/paradigm/.local/share/Steam/ubuntu12_32/libtier0_s.so
[#12](/issue/ValveSoftware/steam-for-linux/12) 0xf7d3cd4c in start_thread () from /lib/i386-linux-gnu/libpthread.so.0
[#13](/issue/ValveSoftware/steam-for-linux/13) 0xf7c7bdde in clone () from /lib/i386-linux-gnu/libc.so.6

This should definitely be low-priority as I doubt it effects overly many people other than myself, but if someone could add a check that the usb requirements are met before trying to use usb I would be very grateful.