Fixes for ZPP string arguments (and stuff)

I stepped through all usages of `zend_parse_parameters()` in all core extensions except for `ext/standard` to make sure the correct type is used for the string length.

At the same time I've looked at overzealous replacements of `int` with `size_t`; some values work with negative numbers and as such should remain as `int`.

Finally, some minor inconsistencies were addressed.

This is a PR for two reasons:
1. Gather more feedback before submission
2. Let it run through Travis CI