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.
Differential Revision: https://reviews.freebsd.org/D37121