Cygwin: pty: Rename fhandler/tty.cc to fhandler/pty.cc.

- Previously, we have two tty.cc, one is winsup/cygwin/tty.cc and
  the other is winsup/cygwin/fhandler/tty.cc. This is somewhat
  confusing. This patch renames fhandler/tty.cc to fhandler/pty.cc.

winsup/cygwin/Makefile.am

@@ -108,7 +108,7 @@ FHANDLER_FILES= \
 	fhandler/tape.cc \
 	fhandler/termios.cc \
 	fhandler/timerfd.cc \
-	fhandler/tty.cc \
+	fhandler/pty.cc \
 	fhandler/virtual.cc \
 	fhandler/windows.cc \
 	fhandler/zero.cc