src/useradd.c: Add fmkomstemp() to fix mode of </etc/default/useradd>

src/useradd.c: Add fmkomstemp() to fix mode of </etc/default/useradd>

The mode of the file should be 644, but mkstemp(2) was transforming it
to 600.

To do this, we need a function that accepts a mode parameter.  While we
don't need a flags parameter, to avoid confusion with mkostemp(2), let's
add both a flags and a mode parameter.

Link: <https://github.com/shadow-maint/shadow/pull/1080>
Reported-by: kugarocks <kugacola@gmail.com>
Suggested-by: kugarocks <kugacola@gmail.com>
Tested-by: kugarocks <kugacola@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>