update os.h to fix filesize() on older win32

_fstat() always fails under older 32bit WinXP/Win2003 targets.

_filelength() just works for both WinXP SDK and later Win7+ 32bit targets.

Fix compilation on Android.

1. Remove lock.
2. Improve error detection.
3. Remove unsupported log levels.

Loglevel per sink. Thanks @facontidavide 

Added the ability to truncate the basic file logger.