chmod: Implement X perm symbol

Instead of clearing the format bits before calling parsemode, leave
them in so we can differentiate between directories and other files,
then clear the format bits in the result.

chmod.1

@@ -47,7 +47,7 @@ If
 .Ar mode
 is
 .Em symbolic
-"[ugoa]*[+-=][rwxst]*"
+"[ugoa]*[+-=][rwxXst]*"
 .Bl -tag -width Ds
 .It u|g|o|a
 owner | group | other (non-group) | everyone
@@ -55,6 +55,8 @@ owner | group | other (non-group) | everyone
 add | remove | set
 .It r|w|x|s|t
 read | write | execute | setuid and setgid | sticky
+.It X
+execute, if directory or at least one execute bit is already set
 .El
 .Sh OPTIONS
 .Bl -tag -width Ds