Add an initial regress test for pkcs7.

This currently fails when signing/verifying with a detached signature.
pk7_doit.c r1.20 introduced a NULL check that ensures that the signature
contents are not NULL, however this breaks detached signature processing.
Fix this by allowing the signature contents to be NULL when operating with
a detached signature.

Found the hard way by sthen@.

ok sthen@
Add support for adjusting the receive filter to allow for promiscuous
mode and reception of multicast traffic.

ok matthieu@ "looks good to me" rapha@
On IP28, ignore (as in, do not report) GIO bus errors where the address isn't
actually part of the GIO address range; these obviously come from speculative
R10000 execution and are completely harmless (except for the need to acknowledge
them, which we have to do anyway).