Rework temp file handling to avoid GNU libc warnings

Original patch provided by Paul Eggert <eggert@cs.ucla.edu>.
GNU libc will generate a link-time warning if we use mktemp() even
though we are using it safely (we only use it with mkfifo()).
Avoid this and clean up some handling.

First, check all calls related to temporary files and exit with a
fatal error and a useful message if we can't obtain them.  In some
situations it might be possible to continue with reduced capability
but it's not worth the effort.

On POSIX systems we can create anonymous temp files using O_TMPFILE
if it's supported, else if we're using the default location and we
have dup(2), we can use standard tmpfile() and get an FD from it.

If we need a named temp file and FILE* and we have mkstemp() we can
use that, else if we have fdopen() we can get a temp FD and open it.
If none of those are available all we can do is generate a temp name
then open it with fopen() which is not secure.

* src/makeint.h (get_tmpdir): Declare it for use elsewhere.
* src/misc.c (get_tmpdir): Make it public not static.
(get_tmptemplate): Simplify the implementation.
(get_tmppath): Only define this if we have to have it to avoid
warnings from GNU libc.
(get_tmpfd): Generate fatal errors on error.
(get_tmpfile): Ditto.  Open files in "wb+" mode to match tmpfile().
Require a filename pointer (all callers want it).
* src/os.h (os_anontmp): Implement for posixos.c as well.
* src/posix.c (jobserver_setup): Don't use mktemp to avoid GNU libc
errors.  Instead construct the FIFO name based on the PID.
(osync_setup): get_tmpfd() can't fail so don't check it.
(os_anontmp): If the system supports O_TMPFILE use it.  If not, and
we want to create the temporary file in the default directory, we
can use tmpfile() then use dup() to copy the file descriptor.
* src/main.c (main): get_tmpfile() can't fail.
* src/vmsjobs.c (child_execute_job): get_tmpfile() can't fail.

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