Even after my earlier commit 989e194 the current implementation still
fails to reject some invalid forms of DECnet syntax:
'decnet net net-10-20-30-0.libpcap.test': no error
'decnet net 1.2.3.0/24': no error
'decnet net 1.2.3.0 mask 255.255.255.0': no error
'decnet net 1.2': no error
Make the verification stricter to solve the remaining part of the
problem: in gen_scode() and gen_mcode() handle DECnet early and reject
all forms, in gen_ncode() handle DECnet early and reject all forms
except "host". Improve various comments to explain the context. For a
number of other "decnet" forms instead of misleading error messages
(e.g., "Mask syntax for networks only") return a more useful error
message:
'decnet net net-10-20-30-0.libpcap.test': 'decnet' is not a valid
 qualifier for 'net'
'decnet net 1.2.3.0/24': 'decnet' is not a valid qualifier for 'net'
'decnet net 1.2.3.0 mask 255.255.255.0': 'decnet' is not a valid
 qualifier for 'net'
'decnet net 1.2': 'decnet' is not a valid qualifier for 'net'

Unlike gen_ncode(), gen_mcode() and gen_mcode6(), there is exactly one
code path into gen_scode(), namely, through an ID in the grammar. This
way, when a comment says "ID -> gen_scode()", it states the obvious, and
when it says "pnum -> gen_scode()", it is wrong, so eliminate this noise
to improve the signal. Add the missing gen_host46_byname() to a couple
code paths.

The existing tests could not detect the recently fixed DECnet bugs
because the definitions in %pqual_features do not distinguish between
different forms of the ID. Then the subsequent foreach loop that
generates invalid "host" primitives does not generate any tests for
protocols that support a form of "host" no matter which forms of ID are
valid and which are not. Then the resulting reject tests have a number
of gaps, including IPv4 IDs in DECnet primitives.
To fix that, in %pqual_features replace the generic "host" with seven
more specific host/net features; in the loop skip the valid permutations
only and include MAC-48, ARCnet and DECnet IDs. The new test space
supersedes a number of earlier reject tests, so remove these. Document
the current code paths in the comments and introduce more helper
functions.