gdb: Allow quoting around string options in the gdb::option framework

gdb: Allow quoting around string options in the gdb::option framework

Currently string options must be a single string with no whitespace,
this limitation prevents the gdb::option framework being used in some
places.

After this commit, string options can be quoted in single or double
quotes, and quote characters can be escaped with a backslash if needed
to either place them within quotes, or to avoid starting a quoted
argument.

This test adds a new function extract_string_maybe_quoted which is
basically a copy of extract_arg_maybe_quoted from cli/cli-utils.c,
however, the cli-utils.c function will be deleted in the next commit.

There are tests to exercise the new quoting mechanism.

gdb/ChangeLog:

	* cli/cli-option.c (parse_option): Use extract_string_maybe_quoted
	to extract string arguments.
	* common/common-utils.c (extract_string_maybe_quoted): New function.
	* common/common-utils.h (extract_string_maybe_quoted): Declare.

gdb/testsuite/ChangeLog:

	* gdb.base/options.exp (expect_string): Dequote strings in
	results.
	(test-string): Test strings with different quoting and reindent.