Problem: new code is duplicated in zsocket and zsock

zsocket is deprecated and there's no reason to add new code to it.
That just creates extra work and maintenance. Those who need this new
functionality can use zsock and the CZMQ v3 API.

Solution: reverted zsocket changes.
Merge pull request #621 from hintjens/master

Problem: new code is duplicated in zsocket and zsock
Problem: zsock defines its own random number helper

zsys already initializes the seed, and czmq_prelude.h provides
randof (n).

Solution: use CZMQ facilities and remove superfluous random number
helper.
Minor clarification in comment
Merge pull request #622 from hintjens/master

Problem: zsock defines its own random number helper
strrchr const correctness. Fixes build in msvc.
Merge pull request #623 from jossgray/master

strrchr const correctness. Fixes build in msvc.
Problem: trex code is flaky

In real use this code breaks; it does not, for instance, return
capture groups correctly. I've switched to another similar piece
of code, SLRE, which was MIT-licensed for v1.0 (has since gone
to GPLv2 which isn't useful for us).

Solution: drop trex code and replace with slre v1.0.

FWIW I also extended the regexp syntax to support \w, \W, \D, \a,
and \A.
Problem: zrex API is a little clunky

Solution: various cleanups to make this more explicit. Since trex
didn't work, we can assume no-one was using this class yet, so the
API change won't break existing applications.
Fixed zrex man page
Updated man pages
Removed debugging code
Merge pull request #624 from hintjens/master

Problem: trex code is flaky
Left another debug output, fixed
Trivial fix to selftest output
Problem: bind code was inelegant

Solution: use zrex for parsing. Combine random and sequential port
scanning into single loop, as they're basically the same work. I've
not tested this to exhaustion.

Note: we can now trivially add randomizaton for other endpoints, and
it might be smarter to return the last bound endpoint as a property
rather than via the return code.
Problem: vsnprintf use for endpoints is clumsy

Solution: use zsys_sprintf and remove size restriction on endpoints.

Note that _attach still uses fixed 256 limit; this should change at
some point to use zrex and no fixed size limit on endpoints.
Problem: WORD is reserved under MSVC

Solution: fixed use of WORD in slre.c; replaced with ALNUM.

Fixes #625.
Updated zsock man page
Merge pull request #626 from hintjens/master

Finishing up with with randomized ephemeral TCP ports