config.setcpv: fix bug #522362

Fix config.setcpv to regenerate USE settings in order to account for
package.env USE settings from the previous package instance.

X-Gentoo-Bug: 522362
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=522362
Acked-by: Alexander Berntsen <bernalex@gentoo.org>
Acked-by: Brian Dolbec <dolsen@gentoo.org>
Run distcc-pump server throughout src_configure() to src_install()

Start distcc-pump server in all phases throughout src_configure() to
src_install() rather than in src_compile() alone. Since each of those
phases may involve some compilations, we should try to take full
advantage of distcc in all of them. Moreover, this silences distcc
warnings about being unable to connect to the server when compiling.

Fixes: https://bugs.gentoo.org/show_bug.cgi?id=507482
Split install_qa_check() into install-qa-check.d scripts

Convert the horrendous install_qa_check() function into a plug-in system
that calls separate QA checking scripts from install-qa-check.d.
Run install-qa-check.d scripts from repositories

---
(following the remaster of install_qa_check())
Allow inheriting utility eclasses in QA checks

Allow inheriting utility eclasses for use in QA checks. The eclasses can
be used e.g. to obtain paths or other data needed for the check.
However, the eclasses must not export phases or set ebuild metadata.