Use g_mkstemp_full instead of mkostemp(s)

Use g_mkstemp_full instead of mkostemp(s)

With g_mkstemp_full, there is no need to distinguish between
mkostemp and mkostemps (no suffix vs. a suffix of a fixed length),
because the GLib function looks for the XXXXXX pattern everywhere
in the string.

Use S_IRUSR | S_IWUSR for the permissions and do not pass O_RDWR
in flags since it's implied.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>