Do bounds checking when unescaping PPP.

Clean up a const issue while we're at it.
Strengthen various bounds etc. checks.

Add more checks, make some checks do a better job of handling too-short
lengths,

Also, rename ldp_msg_print() to ldp_pdu_print(), as it prints a single
PDU, not a single message within a PDU.
Don't run past the snapshot length when doing hex/ASCII dumps.