#80 missing headers in some files

[Mike Frysinger]

exp_clib.c needs unistd.h & sys/wait.h for the funcs it uses

exp_trap.c & pty_termios.c need string.h

pty_termios.c should pull in pty.h when available for openpty()

exp_chan.c should pull in exp_event.h

pty_termios.c should pull in exp_int.h for expErrnoMsg

exp_tty.h should declare exp_tty_cooked_echo() as exp_main_sub.c uses it

[Mike Frysinger]

example changes