Spawn a new thread for cmd-idle rather than relying on problematic pthread timedwait

pthreads-win32 uses GetSystemTimeAsFileTime for *_timedwait (UTC) and has no clock_* at all
winpthreads (mingw-w64) uses _ftime for *_timedwait (local time) and GetSystemTimeAsFileTime for clock_*
Remove now-unused bfg_cond_timedwait which cannot be made portable
Merge commit '15c1bc7' into bfgminer