Introduce eqatag to output proper machine-readable QA logs

The eqatag command syntax conforms to pre-GLEP describing
install-qa-check.d. The qa.log file format strictly conforms to YAML for
easy machine parsing.
Update the QA checks to new eqatag API
Remove __eqalog & __eqawarnlog

Replaced by eqatag.
emerge: check for writable /var/db/pkg (490732)

If there are packages to be merged or unmerge, then bail out early
if /var/db/pkg is not writable (in order to avoid a fatal EROFS error
which would otherwise occur later on). Behavior remains unchanged for
--pretend mode. For --ask mode, it will bail out just after the last
relevant --ask prompt.

In contrast to the writeable_check module, which operates on files just
before they are merged, the new vardbapi.writable check is performed
before anything has been built (much earlier).

X-Gentoo-Bug: 490732
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=490732
Acked-by: Brian Dolbec <dolsen@gentoo.org>