epatch.eclass: Split epatch* logic from eutils

epatch.eclass: Split epatch* logic from eutils

Move epatch and epatch_user (along with the descriptions for all their
variables) into a dedicated epatch.eclass. This function is very
complex, therefore it benefits from separate eclass and a dedicated
maintainer. Furthermore, it is mostly obsoleted by eapply* in EAPI 6.

The new eclass is implicitly inherited by eutils to preserve
compatibility. However, the inherit will be removed in EAPI 7,
and the ebuilds should switch to inheriting epatch directly or using
eapply*.

Thanks to Ulrich Müller for doing the necessary research.