syscall: track syscall system time a bit more explicitly

syscall: track syscall system time a bit more explicitly

Before, it relied on implicit assumptions that syscall-exit event is
right the next one after syscall-enter.  Also, there's some additional
debugging output that might help someone someday.

* count.c (count_syscall): Calculate system time as difference of tcp's
stime and ltime.
* defs.h (struct tcb): Add ltime, atime fields, remove dtime.
* strace.c (droptcb): Print total system time spent by a tcb.
(startup_tcb): Store initial system time in atime.
(next_event): Update stime directly.
* syscall.c (syscall_entering_finish): Store current system time in
tcb's ltime field.
(syscall_exiting_finish): Likewise.