usergen: Replace use of non-standard mktemp(1)

Despite common use in several POSIX operating systems, mktemp(1) is not
defined by POSIX.1-2008, nor even POSIX.1-2017. As long as it is not
introduced, m4(1)'s mkstemp can be used with similar effect.

usergen

@@ -17,6 +17,13 @@ to_bin()
     sed -e 's,\([0-9a-f]\{2\}\),\\\\\\x\1,g' | xargs printf
 }
 
+mktemp_posix()
+{
+    m4 <<EOF
+mkstemp(${TMPDIR:-/tmp}/tmp.XXXXXX)
+EOF
+}
+
 if [ $# != 1 ]; then
     usage >&2
     exit 1
@@ -68,7 +75,7 @@ do
 done
 
 echo >&2
-TMP=$(mktemp)
+TMP=$(mktemp_posix)
 
 cleanup()
 {