gdb: improve command completion for 'print', 'x', and 'display'

The /FMT specification on the print command currently breaks command
completion, so:

  (gdb) p var.<TAB><TAB>
  .... list of fields in var .....

But,

  (gdb) p/d var.<TAB><TAB>
  ..... list of all symbols .....

After this commit this issue is now resolved.

There are some other details around tab-completion and /FMT which
hopefully this commit improves.  So, before:

  (gdb) p/<TAB><TAB>
  .... lists all symbols .....

After:

  (gdb) p/<TAB><TAB>		# Nothing changes...

The thinking here is that after a / the user must type a FMT, but we
don't offer tab completion on FMT characters.  Placing a symbol
directly after a / will not do what the user expects, so offering that
seems wrong.

Similarly, before we had:

  (gdb) p/d<TAB><TAB>
  ... lists all symbols starting with 'd' ....

But afterwards:

  (gdb) p/d<TAB><TAB>		# Adds a single space, so we get:
  (gdb) p/d <CURSOR>

As before, typing a symbol where FMT is expected will not do what the
user expects.  If the user has added a FMT string then upon tab
completion GDB assumes the FMT string is complete and prepares the
user to type an expression.

In this commit I have also added completion functions for the 'x' and
'display' commands.  These commands also support /FMT specifiers and
so share some code with 'print'.

gdb/ChangeLog:

	* printcmd.c: Include 'safe-ctype.c'.
	(skip_over_slash_fmt): New function.
	(print_command_completer): Call skip_over_slash_fmt.
	(display_and_x_command_completer): New function.
	(_initialize_printcmd): Add command completion for 'x' and
	'display'.

gdb/testsuite/ChangeLog:

	* gdb.base/completion.exp: Add new tests.