I believe apm must have been buggy - example output that I found online
showed `tr` was mangling paths with spaces in it. Should be fixed.
Also, use dscl on OS X in __fish_complete_users.fish like
__fish_print_users.fish already does.
This is to the benefit of systems with ancient GNU sed, which does not
recognize "-E", but only "-r".
Fixes#2305 - even if it doesn't replace all `sed -E` invocations in the
codebase, the others are unlikely to occur on CentOS and other similarly
crusty systems.
