gdb: more styling for skip command output

Add function and file styling to output of the 'skip', 'skip file',
and 'skip function' commands.

I did worry a little about this case:

  (gdb) skip -gfile *.c
  File(s) *.c will be skipped when stepping.

After this change the '*.c' will be given file_name_style, even though
it's not an actual filename, but a filename glob.

However, if you do this:

  (gdb) info skip
  Num   Enb Glob File                 RE Function
  1     y      y *.c                   n <none>

Then the '*.c' is already (even before this patch) given
file_name_style, so if nothing else, my change makes things
consistent.  And personally, I think it's OK to style the '*.c'.

There's a similar issue with 'skip -rfunction PATTERN' where the
PATTERN will be styled in the output with function_name_style even
though it's not an actual function name.  As with the filename case
PATTERN is already styled in the 'info skip' table, so I think the
change in this commit is consistent if nothing else.

There should be no functional change after this commit; just improved
styling.

Approved-By: Tom Tromey <tom@tromey.com>