gdb: Use add_setshow_zuinteger_unlimited_cmd in remote.c

gdb: Use add_setshow_zuinteger_unlimited_cmd in remote.c

Switch to use add_setshow_zuinteger_unlimited_cmd for some of the
control variables in remote.c.  The variables
hardware-watchpoint-limit, hardware-breakpoint-limit, and
hardware-watchpoint-length-limit are all changed.  For example, a user
will now see this:

    (gdb) show remote hardware-breakpoint-limit
    The maximum number of target hardware breakpoints is unlimited.

Instead of this:

    (gdb) show remote hardware-breakpoint-limit
    The maximum number of target hardware breakpoints is -1.

And can do this:

    (gdb) set remote hardware-breakpoint-limit unlimited

However, previously any negative value implied "unlimited", now only
-1, or the text "unlimited" can be used for unlimited.  Any other
negative value will give an error about invalid input.  This is a
small change in the user interface, but, hopefully, this will not
cause too many problems.

I've also added show functions for these three variables to allow for
internationalisation.

gdb/ChangeLog:

	* remote.c (show_hardware_watchpoint_limit): New function.
	(show_hardware_watchpoint_length_limit): New function.
	(show_hardware_breakpoint_limit): New function.
	(_initialize_remote): Use add_setshow_zuinteger_unlimited_cmd
	where appropriate, update help text.

gdb/doc/ChangeLog:

	* gdb.texinfo (Remote Configuration): Update descriptions for
	set/show of hardware-watchpoint-limit, hardware-breakpoint-limit,
	and hardware-watchpoint-length-limit variables.