devel/staf: stafproc passes a revoked file descriptor to children

staf's rc script uses daemon(8) to detach stafproc from its controlling
terminal, but it does nothing to stdin. The result is that stafproc has a
stdin in the revoked state, which it passes to child processes. Some
programs can't handle revoked file descriptors, because they return EBADF
despite being opened.

PR: 221161
Approved by: koobs (portmgr)
MFH: 2017Q3
Sponsored by: Spectra Logic Corp