classifier-based ACL: refactor + add output ACL

classifier-based ACL: refactor + add output ACL

For implementation of MACIP ACLs enhancement (VPP-1088), an outbound
classifier-based ACL would be needed. There was an existing incomplete
code for outbound ACLs, it looked almost exact copy of input ACLs, minus
the various enhancements, trying to sync that code seemed error-prone
and cumbersome to maintain in the longer run.

This change refactors the input+output ACLs processing into a unified
routine (thus any changes will have effect on both), and also adds
the API to set the output interface ACL, with the same format
and semantics as the existing input one (except working on output
ACL of course).

WARNING: IP outbound ACL in L3 mode clobbers the ip.* fields
in the vnet_buffer_opaque_t, since the code is using l2_classify.*
The net_buffer (p0)->ip.save_rewrite_length is rescued into
l2_classify.pad.l2_len, and used to rewind the header in case of
drop, so that ipX_drop prints something sensible.

Change-Id: I62f814f1e3650e504474a3a5359edb8a0a8836ed
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>