Fixed bug #644: mongo.ping_interval and mongo.is_master_interval is unused.

The problem was not that they were not used, but only that ini_get() / php -i
didn't show the set value. And this was because we didn't return anything from
the callback functions. We need to return SUCCESS in order for PHP to update
it's internal cache of these values.
Make sure the default INI setting values are used for initialisation as well.
Verify that values are numeric and larger than 0.

INI settings in PHP are *always* strings, and ini_get() will therefore always
return a string as well.

Using is_numeric_string has the interesting property that hexadecimal values (a
la 0x05) are now also supported.
Merged pull request #254