cttyhack: print detected ctty name when called without parameters

cttyhack: print detected ctty name when called without parameters

Sometimes there's a need to figure out the controlling tty from a shell
script, for example, to obtain a line for getty. In this case it's easier
to call cttyhack than trying to repeat some of the cttyhack's logic.

function                                             old     new   delta
cttyhack_main                                        283     327     +44
packed_usage                                       28911   28915      +4

Signed-off-by: Alexander Shishkin <virtuoso@slind.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>