use more apt variable names in various bash functions

Also, remove various unused variables some of which relate to legacy
portage code which has been removed or rewritten.
eapi6: initial EAPI 6 profile stub
eapi6: initial get_libdir implementation

Basically a renamed version of the internal implementation.

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463586
eapi6: unpack() *.txz support

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=458102
eapi6: initial eapply implementation (todo)

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463768
eapi6: initial eapply_user implementation

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=475288
eapi6: initial PATCHES support in src_prepare

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463692
eapi6: mark dohtml as deprecated
eapi6: ban einstall

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=524112
eapi6: econf passes --docdir and --htmldir args

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=468202
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: initial nonfatal die support (todo)

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

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=483244
eapi6: unpack() case-insensitive extension matching

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=476730