Make afs 'false' rather than 'undef' on VMS

Unlike nearly every other Configure variable, the absence of afs
is denoted by a false value rather than undef but we had this
backwards in configure.com. t/op/require_errors.t has been failing
because it checks for equal to 'true' rather than definedness.

configure.com

@@ -5826,7 +5826,7 @@ $ WC "PERL_CONFIG_SH='true'"
 $ WC "_a='" + lib_ext + "'"
 $ WC "_exe='" + exe_ext + "'"
 $ WC "_o='" + obj_ext + "'"
-$ WC "afs='undef'"
+$ WC "afs='false'"
 $ WC "afsroot='/afs'"
 $ WC "alignbytes='" + alignbytes + "'"
 $ WC "aphostname='write sys$output f$edit(f$getsyi(\""SCSNODE\""),\""TRIM,LOWERCASE\"")'"