Use UTF8f in more places

This saves having to allocate as many SVs.
gv.c:gv_fetchmethod_pvn_flags: Avoid unnecessary extra SV
op.c: Refactor calls to bad_type_sv

Every single caller passes gv_ename(namegv), so make it accept a GV
instead and have *it* call gv_ename(namegv).
Stop undef &foo from crashing on lex subs
perldiag: Add the elliptical variant of strict refs error

So that diagnostics.pm can find it.
perldiag: Correct ‘Don't know how to handle magic’
sv.c: Allow COWs through sv_magic unscathed

There is no need to run COWs through sv_force_normal when
attaching magic to them via sv_magic.  (I don’t know about
PERL_OLD_COPY_ON_WRITE, but I left it in, just to be sure.)

Before this, only sv_magicext could attach magic to SVs that were
already COWs.  That meant pos($cow)=7 would leave it as a COW, but
copy-on-write would never happen with tainted strings.
perldiag: Correct ‘Infinite recursion in regex’
perldiag: Document ‘internal %<num>p might conflict’
perldiag: Correct ‘length() used on %s’
perldiag: Document mprotect errors
diag.t: Tolerate trailing spaces in C code
perldiag: Correct ‘Pattern subroutine nesting...’
perldiag: Correct ‘Perl %s required’
perldiag: Correct ‘Repeated format line...’

The perldiag entry has differed from the actual error since both were
added in a1b950687051c.
perldiag: Document ‘SWASHNEW didn't return an HV ref’
perldiag: Correct ‘Unknown Unicode option value’