Solution: rename socket option (and variables and files) from usefd
to use_fd.

Solution: rename socket option (and variables and files) from
pre_allocated_fd to usefd.

Solution: parse the value set by the ZMQ_PRE_ALLOCATED_FD sockopt
when creating a new TCP socket and use it if valid.
Add new tests/test_pre_allocated_fd_tcp.cpp unit test.

Solution: parse the value set by the ZMQ_PRE_ALLOCATED_FD sockopt
when creating a new IPC socket and use it if valid.
Add new tests/test_pre_allocated_fd_ipc.cpp unit test.