devel/libfixposix: Import newer upstream fix

Last fix was not complete, since it didn't take `EAGAIN' semantic
differences between Linux and FreeBSD in consideration.

Reported by: Vasily Postnicov <shamaz.masum@gmail.com>
Obtained from: https://github.com/sionescu/libfixposix/commit/cc50634551849cc667edd5b5b29f6f54ae2f9bc4

Differential Revision: https://reviews.freebsd.org/D33931