Better length assertion hoisting

* src/pf/expand.lua: Eta-reduce consequents to expose information from
  "if" tests.
Fix parsing the accept-all filter

* src/pf/expand.lua: Remove some debug printing.
* src/pf/parse.lua: Parse the empty string to "true".
"tcp port 80" actually is "tcp and port 80"; adapt parser.

* src/pf/parse.lua: Juxtaposition is conjunction, what!