posix: Consolidate register-atfork

Both htl and nptl uses a different data structure to implement atfork
handlers.  The nptl one was refactored by 27761a10 to use a dynarray
which simplifies the code.

This patch moves the nptl one to be the generic implementation and
replace Hurd linked one.  Different than previous NPTL, Hurd also uses
a global lock, so performance should be similar.

Checked on x86_64-linux-gnu, i686-linux-gnu, and with a build for
i686-gnu.