Currently, logrus is used from the Go part of runc init, mostly for a
few debug messages (see setns_init_linux.go and standard_init_linux.go),
and a single warning (see rootfs_linux.go).

This means logrus is part of init implementation, and thus, its setup
belongs to StartInitialization().

Move the code there. As a nice side effect, now we don't have to convert
_LIBCONTAINER_LOGPIPE twice.

Note that since this initialization is now also called from libct/int
tests, which do not set _LIBCONTAINER_LOGLEVEL, let's make
_LIBCONTAINER_LOGLEVEL optional.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

This commit does two things:

1. Consolidate StartInitialization calling logic into Init().
2. Fix init error handling logic.

The main issues at hand are:
- the "unable to convert _LIBCONTAINER_INITPIPE" error from
  StartInitialization is never shown;
- errors from WriteSync and WriteJSON are never shown;
- the StartInit calling code is triplicated;
- using panic is questionable.

Generally, our goals are:
 - if there's any error, do our best to show it;
 - but only show each error once;
 - simplify the code, unify init implementations.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>