remove vestigal threadsv/threadsv_named

the threadsv op and the PL_threadsv_names var were part of the 5.005
threads model, long since removed. Remove some remaining references to
them.
comment pp_foo aliases in pp*.c

Where pp_foo() also handles OP_BAR, add a comment above the function
mentioning that it also does pp_bar.

This means that "grep pp_bar pp*.c" quickly locates the file/function that
handles OP_BAR.
regen/op_private: fix assorted typos
pp.h: remove spurious comment about OPpTARGET_MY

OPpTARGET_MY is unrelated to MAXARG as far as I can tell.
regen/op_private: update TARGLEX comments.

Explain a bit more about how OPpTARGET_MY is used, and remove a temporary
comment I make about inconsistencies in which ops can use the flag as
defined by the old Concise.pm and regen/opcodes T flags.
move OPpTARGET_MY comments from opcode.pl

There is a section in regen/opcode.pl explaining when its okay to
use the T flag / OPpTARGET_MY define.

Move that text to regen/op_private, at the spot where it defines
OPpTARGET_MY. It's more likely to be seen there.
regen/op_private: tidy up rv2cv entry

Remove some temporary comments, and merge the two places where rv2cv flags
are defined.
B.xs: fix syntax err in PERL_VERSION < 17

The blead version of B.xs obviously hasn't been able to be compiled
on old perls for a while. This fixes one such syntax error, blind.