libobs/util:  Add func to get current log handler
libobs/util:  Remove dup. code, add os_opendir

Just use platform-nix.c code for general stuff that mac is compliant
with, and put a define around everything else.  Take that code out of
platform-cocoa.m.

Added os_opendir, os_readdir, and os_closedir to be able to query
available files within a directory.
Add logging to all operating systems

Also have it remove log files past 10 (default, changable with ini
setting as per usual).  10 might be too few.