o add a 'val' element to aliasinfo struct and move -> parse.h

o add a 'val' element to aliasinfo struct and move -> parse.h
o find_alias() now returns an aliasinfo * instead of boolean
o add_alias() now takes a value parameter to store in the aliasinfo.val
o The cmnd, hostspec, runasuser, and user rules now return:
1) positive match
0) negative match (due to '!')
-1) no match
This means setting $$ explicitly in all cases, which I should have done in
the first place.  It also means that we always store a value that is != -1
and when we see a '!' we can set *_matches to !rv if rv != -1.
The upshot of all of this is that '!' now works the way it should in
lists and some of the rules are more uniform and sensible.