Correct ebuild.sh path in man pages, bug #462802
repoman: add --experimental-inherit=<y|n> option

Enable experimental inherit.missing checks which may misbehave when the
internal eclass database becomes outdated.
Exempt /usr/local/portage from repo_name warning.

This will fix bug #248603.
man/portage.5: clarify package.env more
man/portage.5: fix environment spelling
man/make.conf.5: fix documentation spelling
man/make.conf.5: fix substitution spelling
egencache: allow missing --repo if exactly 1 repo
emirrordist: allow missing --repo if exactly one
egencache: handle empty --portdir-overlay