Fix -z display.

Fix -z display.

Before this patch, the following:
    open("qwerty", O_RDONLY)    = -1 ENOENT
    write(2, "wc: qwerty: No such file or dire"..., 38) = 38
was shown totally wrongly with -z:
    open("qwerty", O_RDONLY)    = 38
(yes, that's right, write syscall is lost!)
Now it is shown "less wrongly" as:
    open("qwerty", O_RDONLY <unfinished ...>
    write(2, "wc: qwerty: No such file or dire"..., 38) = 38

* syscall.c (trace_syscall_exiting): Use common TCB_INSYSCALL clearing
via "goto ret". This fixes totally broken display of -z, but even now
it is not working as intended. Add a comment about that.
(trace_syscall_entering): Use common TCB_INSYSCALL setting
via "goto ret".

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>