make.defaults: recurse directories

This makes behavior more consistent with handling of other files, as
discussed in bug #463266.
_autounmask_levels: refactor (no behavior change)
_autounmask_levels: respect keywords, bug #463394

This adds an additional autounmask level which tries to respect
keywords while discarding package.mask as discussed in bug #463394.
PORTAGE_XATTR_EXCLUDE="security.*", bug #461868
Support RESTRICT=splitdebug
getconfig: handle recursive PermissionDenied
make.defaults: profile-formats control recursive
ebuild: standardize error output a bit

A bunch of the messages are written to stdout instead of stderr, and
lack any sort of general context.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>