panic/fork: Command: Do not unwind after fork() in child

panic/fork: Command: Do not unwind after fork() in child

Unwinding after fork() in the child is UB on some platforms, because
on those (including musl) malloc can be UB in the child of a
multithreaded program, and unwinding must box for the payload.

Even if it's safe, unwinding past fork() in the child causes whatever
traps the unwind to return twice.  This is very strange and clearly
not desirable.  With the default behaviour of the thread library, this
can even result in a panic in the child being transformed into zero
exit status (ie, success) as seen in the parent!

Fixes #79740.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>