eapi6: initial einstalldocs implementation

This also adds directory support and the ability to explicitly disable
the install of docs by setting empty variables.

Finally, the default src_install function now calls einstalldocs.

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=459692
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=481980
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463736
eapi6: add global failglob setting support (todo)

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463822
eapi6: ban einstall

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=524112
eapi6: initial nonfatal die support

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=451938
eapi6: initial unpack() absolute path support settings

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=483244
eapi6: initial in_iuse implementation

When no argument is passed it returns a false value similar to how `use`
currently operates. This is because when no argument passed
IUSE_EFFECTIVE is checked if it contains the empty string which it
shouldn't ever.

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=449862