eutils.eclass: Show death notice only when user patches were really applied

eutils.eclass: Show death notice only when user patches were really applied

As part of the user requested feature from [Gentoo-Bug #543878]
eutils.eclass shows a warning regarding user applied patches in case of an
error [Link 1].

However this warning will always be shown even if no user patch were
applied at all (example: empty /etc/portage/<cat>/<pkg> directory).

This commit adds a new global variable "EPATCH_N_APPLIED_PATCHES" which
tracks the number of applied user patches. This allows us to only show the
notice when user patches were really applied.

Link: https://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/eclass/eutils.eclass?r1=1.443&r2=1.444
Gentoo-Bug: https://bugs.gentoo.org/543878