[perl #115736] fix undocumented param from newATTRSUB_flags

flags param was poorly designed and didn't have a formal api. Replace it
with the bool it really is. See #115736 for details.
refactor pp_socket, pp_socketpair, pp_bind

pp_socket: remove unreachable made by commit 9c9f25b8ce

pp_socketpair: increase locality, now gv2/io2 is tested before gv1 is
    processed, *v1 vars become non-const to avoid large WS changes in
    opening new scope

pp_bind: move op_type's init so it isn't saved by CC across a func call
test various types of SVs with call_sv

call_sv takes RVs, PVs, CVs, GVs, and an immortal. This isn't well
documented. CVs and immortals can't, or can't easily be tested from
pure perl, so do it from XS. SVt_PVLV with isGV_with_GP is one thing
call_sv takes but is not tested by this commit. Part of [perl #120826] .
pp.c: Remove redundant diag_listed_as
perldelta: Silence podchecker
Rearrange dump.c to organize docs. Add some perlapi docs for debug methods.