We cannot use blocking file-descriptor I/O on the I/O thread. This
patch adds an assertion to UnixFdWatcher the tests for the O_NONBLOCK
flag, when installing a file descriptor. In UnixFileWatcher, the
Open method tests for the O_NONBLOCK flag for the opened file.
File-descriptor flags for UnixSocketImpl et al are currently set by
UnixSocketImpl itself. Later patches should move this into the
methods of connector classes.
These new classes encasulate file-descriptor watchers, basic file
descriptors, and socket connections. Each class contains callback
methods for it's implemented functionality. Users should inherit
from the classes and overload the callback with their own code.
