calllist: force a arg list, even if empty

fixes test/closures/arg0.pl
p5-siglist: use arg2 as entrypoint

sig_p5 should be the lone entrypoint for p2_sig only, because it parses until end-of-file.
improve arg2 by pushing the single-case n:arg2-name to the end, enabling the other 3 cases.
sigs parse now, but crash in the potion-specific compiler special-cases