libdwfl [2/12]: expose setfunc callback for libdwflP+libebl clients

libdwfl [2/12]: expose setfunc callback for libdwflP+libebl clients

Changes for v6:

- Minor fixes as requested.

Changes for v2:

- No longer exposing this in public libdwfl.h api.

* * *

Renaming pid_set_initial_registers to
__libdwfl_set_initial_registers_thread and adding to libdwflP.h.

This callback was private to one file, but now we need to access it
from the perf_events sampling code as well.

* libdwfl/libdwflP.h (__libdwfl_set_initial_registers_thread): New function.
* libdwfl/linux-pid-attach.c (__libdwfl_set_initial_registers_thread):
  Renamed from pid_thread_state_registers_cb.
  (pid_set_initial_registers): Pass the newly renamed callback.