From libthr, remove special and strange code to set up session and
control terminal, activated when running with pid 1. It is
application duty to handle this, and unsuspecting init replacements
which are linked with libthr would be broken by this.

The pre-resolving of getpid() is restored, just in case.

Reviewed by: jilles
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks