Correct grep regex for POSIX compatibility

Issue brought up by Michael Gmelin in FreeBSD bug:
  https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=263812

The \d patter is not POSIX, instead :digit: or [0-9] should be used.
Opted for the latter.

Resolves #556 from freebsd/freebsd-ports@8e0b9d2

Signed-off-by: Eric F Crist <ecrist@secure-computing.net>

easyrsa3/easyrsa

@@ -2343,7 +2343,7 @@ default_server_san() {
 		awk -F'=' '/^  *CN=/{print $2}'
 		)"
 
-	if echo "$cn" | grep -E -q '^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$'; then
+	if echo "$cn" | grep -E -q '^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$'; then
 		print "subjectAltName = IP:$cn"
 	else
 		print "subjectAltName = DNS:$cn"