Some initial fixed to compile on FreeBSD.

Implement a number of simple fixes to compile on FreeBSD:
- work correctly with a GCC version that doesn't have
  -Wno-unused-result
- compile even if not GNU libc
- tee doesn't have '--append' option on FreeBSD, use
  the short version ('-a').
- fix a typo in log_io.c, ENOENT, not ENONET.