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.

gdb/gdbsupport/common-utils.h

@@ -94,6 +94,16 @@ void string_vappendf (std::string &dest, const char* fmt, va_list args)
 
 char *savestring (const char *ptr, size_t len);
 
+/* Extract the next word from ARG.  The next word is defined as either,
+   everything up to the next space, or, if the next word starts with either
+   a single or double quote, then everything up to the closing quote.  The
+   enclosing quotes are not returned in the result string.  The pointer in
+   ARG is updated to point to the first character after the end of the
+   word, or, for quoted words, the first character after the closing
+   quote.  */
+
+std::string extract_string_maybe_quoted (const char **arg);
+
 /* The strerror() function can return NULL for errno values that are
    out of range.  Provide a "safe" version that always returns a
    printable string.  */