Introduce string_appendf/string_vappendf

Introduce string_appendf/string_vappendf

string_appendf is like string_printf, but instead of allocating a new
string, it appends to an existing string.  This allows reusing a
std::string's memory buffer across several calls, for example.

gdb/ChangeLog:
2017-10-30  Pedro Alves  <palves@redhat.com>

	* common/common-utils.c (string_appendf, string_vappendf): New
	functions.
	* common/common-utils.h (string_appendf, string_vappendf): New
	declarations.
	* unittests/common-utils-selftests.c (string_appendf_func)
	(test_appendf_func, string_vappendf_wrapper, string_appendf_tests)
	(string_vappendf_tests): New functions.
	(_initialize_common_utils_selftests): Register "string_appendf" and
	"string_vappendf tests".