Fix PID prefix printing in "strace -oLOG -ff -p1 -p2 -p3" case

Fix PID prefix printing in "strace -oLOG -ff -p1 -p2 -p3" case

In this case we were printing PIDs to LOG.* files
even though it is not necessary.

The fix is in the addition of "&& followfork < 2" condition.

* strace.c: Remove pflag_seen variable, add print_pid_pfx one.
(process_opt_p_list): Do not pflag_seen++.
(main): Use "nprocs != 0" condition instead of "pflag_seen != 0".
Set print_pid_pfx before entering main tracing loop.
(printleader): Use print_pid_pfx to decide whether to print pid prefix.

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