bsdgrep: Allow "-" to be passed to -f to mean "standard input"

A version of this patch was originally sent to me by se@, matching behavior
from newer versions of GNU grep.

While there have been some differences of opinion on whether stdin should be
closed or not after depleting it in process of -f, I've opted to leave stdin
open and just let the later matching stuff fail and result in a no-match.
I'm not married to the current behavior- it was generally chosen since we
are adopting this in particular from GNU grep, and I would like to stay
consistent without a strong argument to the contrary. The current behavior
isn't technically wrong, it's just fairly unfriendly to the developer-user
of grep that may not realize their usage is trivially invalid.

Submitted by: se