Teach gdb::option about string options

A following patch will make the "pipe" command use the gdb::option
framework for option processing.  However, "pipe"'s only option today
is a string option, "-d DELIM", and gdb::option does not support
string options yet.

This commit adds support for string options, mapped to var_string.
For now, a string is parsed up until the first whitespace.  I imagine
that we'll need to add support for quoting so that we could do:

 (gdb) cmd -option 'some -string'

without gdb confusing the "-string" for an option.

This doesn't seem important for pipe, so I'm leaving it for another
day.

One thing I'm not happy with, is that the string data is managed as a
raw malloc-allocated char *, which means that we need to xfree it
manually.  This is because var_string settings work that way too.
Although with var_string settings we're leaking the strings at gdb
exit, that was never really a problem.  For options though, leaking is
undesirable.

I think we should tackle that for both settings and options at the
same time, so for now I'm just managing the malloced data manually.
It's a bit ugly in option_def_and_value, but at least that's hidden
from view.

For testing, this adds a new "-string" option to "maint
test-settings", and then tweaks gdb.base/options.exp to exercise it.

gdb/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	* cli/cli-option.c (union option_value) <string>: New field.
	(struct option_def_and_value): Add ctor, move ctor, dtor and
	use DISABLE_COPY_AND_ASSIGN.
	(option_def_and_value::clear_value): New.
	(parse_option, save_option_value_in_ctx, get_val_type_str)
	(add_setshow_cmds_for_options): Handle var_string.
	* cli-option.h (union option_def::var_address) <string>: New
	field.
	(struct string_option_def): New.
	* maint-test-options.c (struct test_options_opts): Add default
	ctor and use DISABLE_COPY_AND_ASSIGN.
	<string_opt>: New field.
	(test_options_opts::~test_options_opts): New.
	(test_options_opts::dump): Also dump "-string".
	(test_options_option_defs): Install "string.

gdb/testsuite/ChangeLog:
2019-07-03  Pedro Alves  <palves@redhat.com>

	* gdb.base/options.exp (expect_none, expect_flag, expect_bool)
	(expect_integer): Adjust to expect "-string".
	(expect_string): New.
	(all_options): Expect "-string".
	(test-flag, test-boolean): Adjust to expect "-string".
	(test-string): New proc.
	(top level): Call it.

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