tempfile: add several functions for creating temporary files

tempfile: add several functions for creating temporary files

Add several functions for creating temporary files with
automatically-generated names, analogous to mkstemps(), but also
arranging for the files to be deleted on program exit.

The functions are named according to a pattern depending how they
operate. They will be used to replace many places in the code where
temporary files are created and cleaned up ad-hoc.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>