const + static vtables in threads::shared

const + static vtables in threads::shared

This makes threads::shared have no non-NULL initialized RW static data.
Uninitialized and NULL filled RW data like PL_sharedsv_space and
prev_signal_hook remain, but on some OSes/CCs (Win32 with special tweaks),
this means that now the RW data section in threads::shared shared library
has no disk representation. Static the remaining RW vars to trim the
symbol table on non-Win32.