Fix off-by-one error in supported EAPI list

Fix the off-by-one error in construction of supported EAPI list that
resulted in EAPI 5 being considered unsupported.
Make eapi_is_supported() reuse _supported_eapis list

Make the eapi_is_supported() function use the generated list of
supported EAPIs rather than partial lists and integer comparison.
Support EAPI=6_pre1 for testing EAPI6 features
Add tentative support for EAPI6 --docdir and --htmldir

Pass --docdir and --htmldir to configure scripts that support it.
Add tentative support for EAPI6 get_libdir()

Add get_libdir() function to obtain the basename of libdir using
the same algorithm that econf uses.
Add tentative support for EAPI6 einstalldocs function

Add the einstalldocs function to conveniently install documentation
using the default patterns or DOCS and HTML_DOCS variables.