This is a backport cherry-pick of commits
165156beeb2926472bbceca3c103aacc3f81a8cc
cce974e8ea992d0e6d2f649eca7880b436d91d71
from the v3.0 branch as we need symbol hashes directly inside their
scopes for more general usage than before.
The redefinable keywords must be specified in any .Y file as follows:
toksym: THE_KEYWORD ;