Split out tty and shared_info stuff into their own headers and use throughout.

Split out tty and shared_info stuff into their own headers and use throughout.
Include sys/termios.h for files which need it.
* tty.h: New file.
* shared_info.h: New file.
* fhandler.h: Move inline methods that rely on tty stuff to
fhandler_console.cc.
* fhandler_tty.cc (fhandler_pty_master::process_slave_output): Set
output_done_event immediately after reading data to speed up tty output
processing.
(process_output): Set write_error to errno or zero.
(fhandler_tty_slave::write): Check previous write error prior to writing to
slave end of pipe.  This allows tty output to be slightly less synchronous.
* fhandler_console.cc (fhandler_console::tcsetpgrp): Moved here from
fhandler.h.
(fhandler_console::set_input_state): Ditto.