Use O_EVTONLY and O_SYMLINK open(2) flags found in MacOSX kernel.
Replace O_READONLY with O_EVTONLY and O_NOFOLLOW with O_SYMLINK as later ones
are more suitable for monitoring a files for changes
Fallback to posix flags if darwin`s ones have not been found at compile time.

O_SYMLINK implementation for FreeBSD 11 HEAD r272513 supplied:
patches/freebsd11-O_SYMLINK.patch