Problem: zstr_recvx did not conform to documentation

Solution: return number of strings stored, rather than always
returning 0.
Problem: selftest runs out of ephemeral sockets on TravisCI

Solution: increase ephemeral range from 10 to 50 sockets, in test
case.
Problem: multi-line if statements were misformatted

This stems from an experiment in autoformatting the source code.

Solution: whitespace fixes to revert if statements to prior state.
Problem: search/replace went wrong

Some bogus edits caused syntax errors. Solution: fix these.
Problem: need way to create/check/decode signal messages

This is to allow signals to be sent and mixed with other messages
on sockets, instead of just depending on zsock_signal/_wait.
Merge pull request #862 from hintjens/master

Small fixes + signal support in zmsg