Make threaded execve handling code more reabable and somewhat simpler

Make threaded execve handling code more reabable and somewhat simpler

* strace.c (droptcb): Remove outfname check in "outfname && followfork >= 2" -
with recent changes, followfork >= 2 check guarantees that outfile
was specified, and _is already opened_.
(trace): Move tcb existence check before threaded execve handling.
This allows to remove tcp != NULL checks in threaded execve handling.
Rewrite threaded execve handling code to be less indented,
keeping the same logic.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>