fully implement documented $SIG{__WARN/DIE__} behavior

[mauke]

The documentation for %SIG (in perlvar) states:

The __DIE__ handler is explicitly disabled during the call, so that you can die from a __DIE__ handler. Similarly for __WARN__.

This has never really been true.

There were two basic checks to prevent infinite recursion from a __DIE__ or __WARN__ handler:

1. When an exception is thrown, if $SIG{__DIE__} references a subroutine that is currently active (somewhere on the call stack at the point of the exception), then die() unwinds the stack directly, bypassing the handler. (The same applies mutatis mutandis to $SIG{__WARN__}/warn().)
   This behavior is wrong because the subroutine may have been invoked normally first (i.e. not via the %SIG machinery), so the handler should still kick in. This is bug GH $SIG{__DIE__} handler doesn't work when called explicitly #22984.
   It also causes issues if the subroutine transfers control "sideways" via goto &othersub because then the registered handler is no longer considered "active" even though Perl code is still executing in the context of a __DIE__/__WARN__ handler. Then, if the goto'd &othersub triggers a warning/exception, the __DIE__/__WARN__ handler will be invoked recursively, eventually leading to a C stack overflow. This is bug GH Infinite recursion (+segfault) on die() after goto-ing out of __DIE__ handler #14527.
2. The code for $SIG{__WARN__} (since c5be5b4) and $SIG{__DIE__} (since 8b4094f) mitigates the latter issue by internally unsetting the DIE/WARN hooks for the duration of the handler call.
   Unfortunately, this is not a complete fix because any modification of $SIG{__DIE__}/$SIG{__WARN__} within the handler, even seeming no-ops such as $SIG{__DIE__} = $SIG{__DIE__} or { local $SIG{__DIE__}; }, will reïnstate the internal hooks, thus reärming the __DIE__/__WARN__ handlers. This is bug GH Infinite recursion (+segfault) on warn() after localizing and goto-ing out of __WARN__ handler #22987.

This patch adds two interpreter-global variables that record whether we are currently executing a __DIE__/__WARN__ handler. This fully replaces the old heuristics by a precise check that prevents recursive handler invocation and nothing more.

Exporter::Heavy had to be patched because it relied on the old (buggy) behavior: It registered a $SIG{__WARN__} handler that would reässign $SIG{__WARN__} and then call warn(), expecting the new handler to be called (i.e. two (nested) warn hooks to be active simultaneously). This is no longer possible with the new implementation.

Fixes #22984, #22987.

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• This set of changes requires a perldelta entry, and it is included.