mktemp: add -p/--tmpdir argument

This matches other mktemp implementations, including OpenBSD and GNU.
The -p option can be used to provide a tmpdir prefix for specified
templates. Precedence works out like so:

-t flag:

• $TMPDIR
• -p directory
• /tmp

Implied -t flag (no arguments or only -d flag):

• -p directory
• $TMPDIR
• /tmp

Some tests have been added for mktemp(1) in the process.

Reviewed by: imp (earlier version), wosch
Sponsored by: Klara, Inc.

(cherry picked from commit ac6f924e1cd1ebd9832227e906ebc03e3ba5eede)
(cherry picked from commit d22f03636be1c012951f744f414b0d410970808e)