procedure-documentation works on RTL procedures

procedure-documentation works on RTL procedures

* libguile/procprop.h:
* libguile/procprop.c (scm_procedure_documentation): Move here from
  procs.c, and to make the logic more similar to that of procedure-name,
  which allows RTL programs to dispatch to rtl-program-documentation.

* libguile/programs.c (scm_i_rtl_program_documentation):
* libguile/programs.h:
* module/system/vm/program.scm (rtl-program-documentation): New
  plumbing.

* module/system/vm/debug.scm (find-program-docstring): New interface to
  grovel ELF for a docstring.