Add flag to disable inital reboot(8) userland sync

Add -N flag to reboot(8) which bypasses the userland sync(2) during
reboot(8) while still allow the kernel sync during the reboot(2) syscall
to occur.

An example use of this is when rebooting with disconnected iSCSI sessions
which would otherwise cause the reboot to hang on BIOs that will never
complete.

Reviewed by: bjk
MFC after: 2 weeks
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D4449