gstrfuncs: Add replacement for string-to-number functions

Very often when we want to convert a string to number, we assume that
the string contains only a number. We have g_ascii_strto* family of
functions to do the conversion but they are awkward to use - one has
to check if errno is zero, end_ptr is not NULL and *end_ptr points to
the terminating nul and then do the bounds checking. Many projects
need this kind of functionality, so it gets reimplemented all the
time.

This commit adds some replacement functions that convert a string to a
signed or unsigned number that also follows the usual way of error
reporting - returning FALSE on failure and filling an error output
parameter.

docs/reference/glib/glib-sections.txt

@@ -1475,6 +1475,15 @@ g_ascii_dtostr
 g_ascii_formatd
 g_strtod
 
+<SUBSECTION>
+GNumberParserError
+G_NUMBER_PARSER_ERROR
+g_ascii_string_to_signed
+g_ascii_string_to_unsigned
+
+<SUBSECTION Private>
+g_number_parser_error_quark
+
 <SUBSECTION>
 g_strchug
 g_strchomp