Enhance get_tmpfile() and add get_tmppath()

Enhance get_tmpfile() and add get_tmppath()

Move all the logic on creating temporary files into misc.c, and add
a new function get_tmppath() that returns the pathname to a temporary
file without creating or opening it.

* src/makeint.h: Add a declaration for get_tmppath().  Remove the
template argument from get_tmpfile(): it will compute its own.
* src/main.c (main): Remove the logic for computing templates.
* src/vmsjobs.c (child_execute_job): Ditto.
* src/misc.c (get_tmptemplate): New function to return an allocated
template string for use with various mktemp-style functions.
(get_tmppath): Return an allocated path to a temporary file, but do
not create it.  Generally this should be avoided due to TOCTOU issues.
(get_tmpfile): Use get_tmptemplate() to generate a template rather
than using one passed in.  If we don't have mkstemp() then use
get_tmppath() to compute the path of a temp file.