• Remove SEM option by making it always avalible

Since FreeBSD 8.x EOL in 2015-08-01 the option doesn't have any value beyond
footshooting[1]. sem_open() and sem_init(pshared=1) always work FreeBSD 9.0
or later after base r201546.

[1] https://lists.freebsd.org/pipermail/freebsd-ports/2017-April/108116.html

PR: 218641
Reported by: jbeich
Exp-run: antoine
MFH: 2017Q2
Differential Revision: https://reviews.freebsd.org/D10446