Fix PR gdb/17820

Fix PR gdb/17820

This patch is a comprehensive fix for PR 17820 which reports that
using "set history size unlimited" inside one's gdbinit file doesn't
really work.

There are three small changes in this patch.  The most important change
this patch makes is to decode the argument of the "size" subcommand
using add_setshow_zuinteger_unlimited_cmd() instead of using
add_setshow_uinteger_cmd().  The new decoder takes an int * and maps
unlimited to -1 whereas the old decoder takes an unsigned int * and maps
unlimited to UINT_MAX.  Using the new decoder simplifies our handling of
unlimited and makes it easier to interface with readline which itself
expects a signed-int history size.

The second change is the factoring of the [stifle|unstifle]_history logic
into a common function which is now used by both init_history() and
set_history_size_command().  This is technically the change that fixes
the PR itself.

Thirdly, this patch initializes history_size_setshow_var to -2 to mean
that the variable has not been set yet.  Now init_history() tests for -2
instead of 0 to determine whether to give the variable a default value.
This means that having "set history size 0" in one's gdbinit file will
actually keep the history size at 0 and not reset it to 256.

gdb/ChangeLog:

	PR gdb/17820
	* top.c (history_size_setshow_var): Change type to signed.
	Initialize to -2.  Update documentation.
	(set_readline_history_size): Define.
	(set_history_size_command): Use it.  Remove logic for handling
	out-of-range sizes.
	(init_history): Use set_readline_history_size().  Test for a
	value of -2 instead of 0 when determining whether to set a
	default history size.
	(init_main): Decode the argument of the "size" command as a
	zuinteger_unlimited.

gdb/testsuite/ChangeLog:

	PR gdb/17820
	* gdb.base/gdbinit-history.exp: New test.
	* gdb.base/gdbinit-history/unlimited/.gdbinit: New file.
	* gdb.base/gdbinit-history/zero/.gdbinit: New file.