don't export usex only for EAPI >= 5

Previously it wasn't exported at all so ebuilds with EAPI < 5 using usex
from the eutils eclass would break, e.g. sys-apps/busybox.
default to PORTAGE_COMPRESS=bzip2 matching portage
add more todo tasks
remove old, misspelled var from export skip list
minor spelling fix: seperate -> separate
minor comment spelling fix