s3:winbindd: fix problems with SIGCHLD handling (bug #7317)

s3:winbindd: fix problems with SIGCHLD handling (bug #7317)

The main problem is that we call CatchChild() within the
parent winbindd, which overwrites the signal handler
that was registered by winbindd_setup_sig_chld_handler().

That means winbindd_sig_chld_handler() and winbind_child_died()
are never triggered when a winbindd domain child dies.
As a result will get "broken pipe" for all requests to that domain.

To reduce the risk of similar bugs in future we call
CatchChild() in winbindd_reinit_after_fork() now.

We also use a full winbindd_reinit_after_fork() in the
cache validation child now instead instead of just resetting
the SIGCHLD handler by hand. This will also fix possible
tdb problems on systems without pread/pwrite and disabled mmap
as we now correctly reopen the tdb handle for the child.

metze