Allow USERNAME to be specified on the command line.

There are some systems that have usernames of the form DOMAIN\username,
which causes an invalid escape character to be inserted.  I was going to
add some escaping, but decided it would be best to just outright set the
value, since I don't want the DOMAIN portion in there anyways.
Remove proto as an include directory.

The proto directory was removed in 0ef90c13b72b74928bfb3c183c7a5bd7240b51ad.
Remove sizeof(int) < 4 related code
cleanup os_unixx.h
inline the rest of os_unixx.h into os_unix.c