Switch sending of messages from send to writev.
Wrap writev system call for unit testing.
Always build unit tests with debug symbols.
Implement fake_writev for unit testing.
Ensure that the copied path is zero terminated.
Implement fake_writev for unit testing.
Remove some const.

Unfortunately,  iovec.iov_base is not const, so there is no way to preserve the const.
Remove some const.

This is done because parse_message no longer uses a const parameter.
Implement fake_writev for unit testing.