Add custom (faster) vfprintf implementation (disabled by default)

* defs.h: Declare strace_vfprintf either as a alias to vfprintf
or as a bona fide function. USE_CUSTOM_PRINTF define controls whether
we use strace_vfprintf. By default, we don't.
* strace.c (tprintf): Call strace_vfprintf instead of vfprintf.
* vsprintf.c: New file, implements strace_vfprintf.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
4 files changed