Problem: need to support ZMQ_MAX_SOCKETS

Solution: added zsys_set_max_sockets, and ZSYS_MAX_SOCKETS environment
variable. Note that currently the application cannot set max_sockets
after creating a socket (it will assert, not return an error).

Additionally, changed zsys_set_iothreads to zsys_set_io_threads for
consistency with ZMQ constants.
Minor cosmetic fixes