Whitespace fixes
Thread local storage on gcc uses __thread
Problem: _test methods are not consistent

CLASS RFC specifies void methods; if a test fails the code should
abort. There is no semantic for returning an error code. However
many of the classes used an int type for their test method.

Solution: change all those classes to use a void method.
Added zfile_has_changed() method & test
Updated man pages
Problem: need to reload config file when changed

Solution: zconfig_has_changed() and zconfig_reload() can be called
from in a daemon at regular intervals to reload a configuration
tree from its original file location, when changed externally.
Problem: need access to last-loaded config filename

Specifically, for logging reload operations.

Solution: zconfig_filename().
Finished initial zactor implementation

Problem: zthread API too complex and not consistent with CLASS
standard. New zsock approach removes need to pass zctx down the
chain. Removal of zctx offers a simpler API, which zactor uses.

A zactor has a specific start-up and shutdown protocol, using
messages over the PAIR-PAIR pipe. This standardizes emerging good
practice for attached threads.
Merge pull request #474 from hintjens/master

zconfig_reload(), zactors, and other great stuff