Problem: zproto_client_c borks if method body is empty

It uses this to generate a comment header.

Solution: use a default.
Problem: does not generate correct methods in some cases

Solution: if method doesn't have an explicit 'return' attribute,
return 0. We don't use void methods, so the API can consistently
report an interrupt with -1 or NULL.
Problem: various errors in code generators

- did not use UPPERCASE-STYLE for protocol commands - fixed
- did not generate stubs for actions in "*" events - fixed
- used C: and S: in command descriptions rather than grammar - fixed

Mostly, driven by needs of Malamute Protocol.
Merge pull request #123 from hintjens/master

Small incremental improvements