linux(4): Check fd passed to unlockpt()

In our implementation, grantpt() and unlockpt() don't actually have
any use, because PTY's are created on the fly and already have proper
permissions upon creation.
Atleast check that a proper fd passed to unlockpt(). For grantpt()
Glibc calls TIOCGPTN ioctl which would fail if fd is not a master.

Differential Revision: https://reviews.freebsd.org/D40100
MFC after: 1 week