rewrote posix_net.cpp based on win_net.cpp

And suddenly hosting a server on linux works as good (or bad)
as on windows.
Even though there are still strange bugs (massive lags in one
direction), at least it works at all now.
Configurable username for Multi-Player (ui_name), com_pause no cheat

D3BFG gets the username from Steam, in the GPL release it just
uses the hostname.
Now it's possible to set a custom name with the ui_name CVAR
(like in classic doom3). If ui_name is empty ("") the hostname
is used.

The window losing focus in MP resulted in the console being spammed
with "changing com_pause not allowed in Multi Player" (or similar)
messages. Added CVAR_NOCHEAT flag to com_pause to get rid of that.
Some timer-related improvements for POSIX

* idSysSignal::Wait should now handle timeouts > 1s better (or at all)
* Use clock_gettime for Sys_Milliseconds()
* Use CLOCK_MONOTONIC_RAW (and CLOCK_MONOTONIC as fallback if
  that's not available) for Sys_Milliseconds() and Sys_Microseconds()
  CLOCK_MONOTONIC can behave strange when NTP is used
* Small fixes in Sys_Microseconds(): use 64bit int to store time
  (like return type...), divide by 1000 (not 1000000) for nsec=>musec