Make it compile with SDL2 again

For some reason SDL.h (or headers included by it) need some
string functions (like strncmp) in inline-functions (that we
don't even use).
Str.h has #defines preventing their usage.. so #undef those in
the (few) sourcefiles that need SDL headers
Improve POSIX threading code

* setting threadname is now done in a seperate function so it's a bit
cleaner (it's different for every platform..)
* replace/refactor signaling code (based on my SDL threading branch and
  the old pthread signaling code from RB):
  - The interface is like on win32 now (Sys_Signal* functions instead of
    overwriting idSysSignal class)
  - created a custom signalHandle_t struct for that, which contains all
    needed information
  - Mimic Windows functions used in win32 implementation more closely,
    e.g. signal all waiting threads on manualReset signalRaise, count
    waiting threads etc. I'm pretty sure the behavior on Win32 and POSIX
    now is identical (as far as possible).
Refactor setting threadname on POSIX systems

* setting threadname is now done in a seperate function
* if setting the threadname fails, it just prints a warning now
  instead of terminating the game with a FatalError
change threadnames so they fit into 15chars

+ terminating null byte - that's the limit of threadnames on linux

Furthermore: idJobThread::Start used va() to create the threadname.
va() isn't threadsafe... so I replaced it with a local buffer and
idStr::snPrintf()
Renamed in_kbd to in_keyboard
Merge remote-tracking branch 'DanielGibson/improve-threading'
Added pthread priority code but it is disabled by default because it requires root privileges