Add tentative EAPI6 nonfatal support to die()

Add support for die() to respect 'nonfatal' modifier if
'--respect-nonfatal' (or '-n') option is used. This allows eclasses
to create custom ebuild helpers that mimic built-in helper behavior.
Add tentative EAPI6 in_iuse() function

Add a function to query IUSE_EFFECTIVE for flags.
Add tentative EAPI6 phase functions
Disallow helpers in global scope in EAPI 6

Disallow calling most of the ebuild helpers in global scope since they
are meaningless in that context. Most of them are also prohibited by PMS
for all EAPIs, so EAPI 6 sounds like a good moment to finally enforce
that restriction.
Ban einstall for EAPI 6
Deprecate dohtml for EAPI 6