Increase the default mlock limit from 64k to 256k

The current default has been unchanged for 14 years.  Increase it to
keep pace with modern hardware and software.  security/pinentry-gnome,
in particular, can sometimes need 112 kB.

PR:		297452
MFC after:	2 weeks
Sponsored by:	ConnectWise
Reviewed by:	cye, emaste
Differential Revision: https://reviews.freebsd.org/D58811

usr.bin/login/login.conf

@@ -31,7 +31,7 @@ default:\
 	:cputime=unlimited:\
 	:datasize=unlimited:\
 	:stacksize=unlimited:\
-	:memorylocked=64K:\
+	:memorylocked=256K:\
 	:memoryuse=unlimited:\
 	:filesize=unlimited:\
 	:coredumpsize=unlimited:\