Add support for FEATURES=parse-eapi-ebuild-head, which is similar to GLEP 55

Add support for FEATURES=parse-eapi-ebuild-head, which is similar to GLEP 55
except that the EAPI is parsed from the head of the ebuild (first 30 lines).
This feature is only intended for experimental purposes and should not be
enabled under normal circumstances.

svn path=/main/trunk/; revision=13173