perldelta: typo
Stop creating defelems for undef in foreach(@_)

This is part of ticket #119433.  This particular bug is triggered by
Data::Dump’s test suite.

Commit ce0d59f changed arrays to use NULL for nonexistent elements,
instead of &PL_sv_undef (the special scalar returned by Perl’s ‘undef’
operator).

‘foreach’ was not updated to account.  It was still treating
&PL_sv_undef as a nonexistent element.  This was causing ‘Modifica-
tion of non-creatable array value attempted, subscript 0’, due to a
similar bug in vivify_defelem, which the next commit will fix.

(Fixing vivify_defelem without fixing foreach will make the test pass,
but for foreach to create a defelem to begin with is inefficient and
should be addressed anyway.)
Make vivify_defelem allow &PL_sv_undef array entries

This is something I failed to change in commit ce0d59f.  I don’t know
of a way to trigger this in pure-Perl code, hence the use of XS in
the test.  It did show up in pure-Perl code due to a bug fixed by the
previous commit.
Increase $XS::APItest::VERSION to 0.56
perlpacktut: mention h2ph

Mentioning syscall.ph without saying how to get it is not
as helpful as it could be.
perldiag: Reword/rewrap embedded null warning

‘syscall arguments’ implies arguments to the syscall() function,
which is not the case here.  Also reword the rest to sound better
in English.
inline.h: Avoid redundant ckWARN call

ckWARN calls Perl_ckwarn.  So does Perl_ck_warner.  So there is no
need to use both.