Switch to using poll(2) in place of select(2).

I've always thought poll was more hassle to set up, because if you
want to reuse part of your pollfds list between calls then you have to
index every fd by its position in the list as well as the fd number
itself, which gives you twice as many indices to keep track of than if
the fd is always its own key.

But the problem is that select is fundamentally limited to the range
of fds that can fit in an fd_set, which is not the range of fds that
can _exist_, so I've had a change of heart and now have to go with
poll.

For the moment, I've surrounded it with a 'pollwrapper' structure that
lets me treat it more or less like select, containing a tree234 that
maps each fd to its location in the list, and also translating between
the simple select r/w/x classification and the richer poll flags.
That's let me do the migration with minimal disruption to the call
sites.

In future perhaps I can start using poll more directly, and/or using
the richer flag system (though the latter might be fiddly because of
sometimes being constrained to use the glib event loop). But this will
do for now.

Recipe

@@ -288,7 +288,7 @@ MISCNETCOMMON = timing callback MISC version tree234 CONF
 MISCNET  = MISCNETCOMMON be_misc settings proxy
 WINMISC  = MISCNET winstore winnet winhandl cmdline windefs winmisc winproxy
          + wintime winhsock errsock winsecur winucs miscucs winmiscs
-UXMISCCOMMON = MISCNETCOMMON uxstore uxsel uxnet uxpeer uxmisc time
+UXMISCCOMMON = MISCNETCOMMON uxstore uxsel uxpoll uxnet uxpeer uxmisc time
          + uxfdsock errsock
 UXMISC   = MISCNET UXMISCCOMMON uxproxy uxutils
 
@@ -365,6 +365,7 @@ PUTTYGEN_UNIX = sshrsag sshdssg sshprime sshdes ARITH sshmd5 version sshprng
          + sshrand uxnoise sshsha MISC sshrsa sshdss uxcons uxstore uxmisc
          + sshpubk sshaes sshsh256 sshsh512 IMPORT puttygen.res time tree234
          + uxgen notiming CONF sshecc sshecdsag uxnogtk sshauxcrypt sshhmac
+         + uxpoll
 puttygen : [U] cmdgen PUTTYGEN_UNIX
 cgtest   : [UT] cgtest PUTTYGEN_UNIX