Move string_to_uint* functions to a separate file

Move string_to_uint* functions to a separate file

Make string_to_uint* functions available to other binaries
included in the strace package.

* string_to_uint.h: New file.
* defs.h: Include it.
(string_to_uint_ex, string_to_uint_upto, string_to_uint): Move
to string_to_uint.h.
* util.c (string_to_uint_ex, string_to_uint): Move ...
* string_to_uint.c: ... to the new file.
* Makefile.am (strace_SOURCES): Add string_to_uint.h and string_to_uint.c.

Signed-off-by: Edgar Kaziakhmedov <edgar.kaziakhmedov@virtuozzo.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>