src/useradd.c: Add fmkstemp() to fix file-descriptor leak

src/useradd.c: Add fmkstemp() to fix file-descriptor leak

This function creates a temporary file, and returns a FILE pointer to
it.  This avoids dealing with both a file descriptor and a FILE pointer,
and correctly deallocating the resources on error.

The code before this patch was leaking the file descriptor if fdopen(3)
failed.

Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>