Simplify various error reporting.

Simplify various error reporting.

In gen_linktype() instead of many bpf_error() calls and hard-coded
messages have one catch-all call at the end with one parametrised
message; move the DLT description to the end of the message.  In
gen_proto_abbrev_internal() have one catch-all call at the end with one
parametrised message.  In gen_load_internal() include the keyword in the
error message.