populate PATH with the current EAPI downwards for helpers

Instead of from 0 to the current pkg EAPI. This way overridden commands
will select the expected, more recent command.
execute the original, nondeprecated helper without munging PATH

This fixes the potential case in which the deprecated helper calls other
helpers that are either new or overridden in the current EAPI.
remove exec privs from setup.py
eapi6: initial EAPI 6 profile
eapi6: initial get_libdir implementation

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=463586
eapi6: add .txz unpacking support

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

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: add support for econf/einstall --docdir and --htmldir (todo)

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=468202
eapi6: initial einstalldocs implementation

HTML_DOCS support might be tacked on later if that becomes part of the
spec.

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=459692
eapi6: add directory support for DOCS

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=481980
eapi6: add support for explicitly disabling DOCS install

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 (todo)

X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=483244
eapi6: call einstalldocs from default_src_install
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.