stty: fix setting of 'extproc' on BSD

stty: fix setting of 'extproc' on BSD

This setting is unusual on BSD as it's read normally in the local
flags returned by tcgetattr(), but can only be set with an ioctl.
Setting with tcsetattr() is ignored.

* src/stty.c (NO_SETATTR): A new flag to indicate the setting
is read and displayed like a normal termios flag, but is set
in some other manner.
(main): Skip tcsetattr() for this setting when this flag is set.
Also fixup the exiting 'extproc' processing to handle the
'-extproc' case correctly.
(sane_mode): Skip setting '-extproc' for 'sane' to avoid the error.
This isn't ideal but matches the operation of the BSD native stty.