Introduce string_vprintf

Introduce string_vprintf

This adds string_vprintf, a va_list variant of string_printf.  This
will be used in later patches.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* unittests/common-utils-selftests.c (format): New function.
	(string_vprintf_tests): New function.
	(_initialize_common_utils_selftests): Register new tests.
	* common/common-utils.c (string_vprintf): New function.
	* common/common-utils.h (string_vprintf): Declare.