Line control
------------
The line control functions are not really supported by old Minix tty
drivers, and the fake versions in this implementation can not do much
to get around this.  It may be best to leave the following modules out
of the library:

    tcdrain.$O
    tcflow.$O
    tcflush.$O
    tcsendbreak.$O

Then the linker will complain about missing routines, and you can look at
these routines to decide whether they are good enough for a particular
application.  tcflush() works in some cases.

None of these routines is POSIX as POSIX doesn't specify ENOSYS as a valid
return value for any of them.