Add bup-features to report status; drop ACL warnings
Add a new "bup features" command that reports information about the
status and capabilities of the current installation.
Given that, and the new summary at the end of the ./configure output,
drop the platform specific warnings, which were a bit odd anyway,
since they wouldn't warn you on say Linux if we found libacl, but it
didn't actually have all the features we required.
Signed-off-by: Rob Browning <rlb@defaultvalue.org> Tested-by: Rob Browning <rlb@defaultvalue.org>