Added uxsel.c, into which I've moved those parts of the uxnet.c

Added uxsel.c, into which I've moved those parts of the uxnet.c
functionality that deal with selectable fds in general. The idea is
that pty.c will stop passing its fd straight to pterm.c and hand it
to this module instead, and pterm.c will start requesting a general
list of fds from this module rather than expecting a single one from
pty.c, with the ultimate aim of pterm.c being able to form the basis
of a Unix PuTTY as well as pterm proper.

git-svn-id: http://svn.tartarus.org/sgt/putty@3015 cda61777-01e9-0310-a592-d414129be87e