exec: simplify tempfile error msg handling

Signed-off-by: Steve Bennett <steveb@workware.net.au>
bootstrap: set HAVE_MKSTEMP on unix

All reasonable unix platforms have mkstemp(), so
use it in bootstrap jimsh.
Without it, exec is non-functional.
(Previously it was just assumed to exist in [exec], but not [file tempfile])

Signed-off-by: Steve Bennett <steveb@workware.net.au>