Support checking for protocols specified by an LLC SAP on FDDI, Token

Ring, and RFC 1483-style ATM, as well as on Ethernet.

Support checking for LLC SAP protocols other than OSI protocols on
Ethernet - for now, we check only the DSAP on those, rather than
checking both the DSAP and SSAP as we do for OSI, as I think, in some
cases, the SSAP isn't the same as the DSAP.

When generating protocol type checks on link-layer types with no type
field, where packets are always IP (SLIP, BSD/OS SLIP, raw IP), generate
a "test" that always succeeds if the protocol being checked for is IP or
IPv6 and a "test" that always fails otherwise.  (We originally did
"gen_true()" if the protocol is IP, and bogusly generated code to check
the field at an offset of -1 otherwise; a subsequent change caused us
always to do "gen_true()", but that doesn't properly handle attempts to
check for other protocols - those attempts should generate code that
always fails, meaning that if you try to look for ARP packets in such a
capture the BPF compiler will return "expression rejects all packets" as
an error - and still generated extra code not all of which was removed
by the optimizer.  The current code generates no *more* BPF code.)

Add "stp", which checks for the LLC SAP for the Spanning Tree Protocol.

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