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

(cherry picked from commit 0f62290e7f749b7a453d413dd72c406129a9aba5)