Add parse_uint function to improve integer overflow handling

Originally found by oss-fuzz (issue 525) in get_ansi_color using ubsan.
After a lot of analysis I'm 99% sure this isn't security relevant so
it's fine to handle this publicly.

The fix is mainly adding a function that does it right and use it
everywhere. This is harder than it seems because the strtol() family of
functions doesn't have the friendliest of interfaces.

Aside from get_ansi_color(), there were other pieces of code that used
the same (out*10+(*in-'0')) pattern, like the parse_size() and
parse_time_interval() functions, which are mostly used for settings.
Those are interesting cases, since they multiply the parsed number
(resulting in more overflows) and they write to a signed integer
parameter (which can accidentally make the uints negative without UB)

Thanks to Pascal Cuoq for enlightening me about the undefined behavior
of parse_size (and, in particular, the implementation-defined behavior
of one of the WIP versions of this commit, where something like signed
integer overflow happened, but it was legal). Also for writing
tis-interpreter, which is better than ubsan to verify these things.

src/core/misc.h

@@ -71,6 +71,7 @@ int expand_escape(const char **data);
 int nearest_power(int num);
 
 /* Returns TRUE / FALSE */
+int parse_uint(const char *nptr, char **endptr, int base, guint *number);
 int parse_time_interval(const char *time, int *msecs);
 int parse_size(const char *size, int *bytes);