Feature: libcrmcommon: New pcmk__g_strcat() function

Appends each string in a NULL-terminated list of strings to a
destination buffer using g_string_append(). With -O2, a series of
g_string_append() calls is more efficient than a single
g_string_append_printf() call due to the overhead of formatted output.

So this:

g_string_append_printf(buf, %s=%s, name, value);

becomes this:

pcmk__g_strcat(buf, name, "=", value, NULL);

Ref T152

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>