run tincd from the same directory as tincctl and pass all options to it

For tincctl start, run tincd from dirname($0) not SBINDIR -
this allows painless alternative directory installation and
running from build directory too.

Also while at it, pass the rest of command line to tincd, not
only options before "start" argument.  This way it's possible
to pass options to tincd like this:
  tincctl -n net start -- -d 1 -R -U tincuser ...

And also add missing newline at the end of error message there.

Signed-Off-By: Michael Tokarev <mjt@tls.msk.ru>