Made check_false_ non-template
Simpler tests & fixes

- Fixed TCP send to send FIN with op_fin.
- Fixed TCP4/6 accept to ignore address output parameter if accept operation
  fails.
- Made connection unit tests single-threaded.
Nonblocking TCP receive test

- Added nonblocking TCP receive test.
- Fixed TCP receive operation to ignore "all" flag when "nonblock" flag is
  specified.
- Fixed TCP receive operation to return when FIN is received.
- Added less-than and greater-than check macros.