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).