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.