Fix: combination of --plugin and -f - fails

Fix: combination of --plugin and -f - fails

scenario: you want to remote-control fetchmail, but you don't want to write
passwords into files, so you feed fetchmail a minimal rcfile via stdin with -f
-. this by itself works fine. if you also want or need to use a --plugin (eg.
socat for socks), then things fail badly: the plugin is run without a stdin fd,
hence can't take input from fetchmail, lots of fun ensues.  plugins without -f
- work fine, it's just the combination that fails.

explanation: the root cause is rcfile_y.y, line 493, which closes whatever fd
carried the rcfile. with -f - this closes fetchmail's stdin - and so far that's
unproblematic.  however, in socket.c lines 166ff things go wrong: fetchmail
sets up the plugin with a socketpair, which will likely include the first
unused fd - and fd zero is now indeed unused.  in line 180ff a dup2 replumbing
from "that fd" (=zero) to zero is performed - and then "that fd" is closed.
and hey presto, we've got no fd zero = stdin for the plugin.

solution: the simplest solution (patch attached) is to make the fclose of the
rcfile conditional, ie. don't close if it's stdin. in the long run the
dup2+close code might be made more robust by not doing a dup2+close if fd[0] is
already 0 or 1.