libbase: logging fixes

Win32:

- getprogname(): call basename() which is available in mingw's crt.
  Don't potentially go recursive with DCHECK_GT().

- Use Win32 critical section instead of mutex.

Other:

- Change log_characters check to compile-time.

- Fix code that gets the basename of __FILE__. The previous code was not
  setting _file, so it didn't work.

- Save and restore errno for LOG calls. Inspired by similar Chromium code.

Change-Id: Ie7bb700918be726fa81d60177d1894d2daeff296
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>