mktemp: synchronize the -p option with docs

mktemp: synchronize the -p option with docs

* src/mktemp.c (usage): Synchronize the -p option description with
the logic and info docs.  I.E. that -p is just an alias of --tmpdir.
Also for consistency treat --tmpdir='' the same with or without -t.
I.E. always ignore the --tmpdir option if the param is empty.
Fixes http://bugs.gnu.org/15425