core: add compile time format check to format_string_std

Had to rewrite the thing, because gcc's warnings don't work
with templatized var-args. Since there is no string-format.cpp
and I didn't want to inline it, moved it to format.cpp.

String formatting is distributed around at least four
headers: membuffer.h, subsurface-string.h, format.h
and format-string.h. This really should be unified!

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/format.h

@@ -25,6 +25,9 @@ __printf(2, 0) int vasprintf_loc(char **dst, const char *cformat, va_list ap);
 
 #ifdef __cplusplus
 }
+
+__printf(1, 2) std::string format_string_std(const char *fmt, ...);
+
 #endif
 
 #endif