pr4225 (user-space probing prototype) merged into mainline

[Frank Ch. Eigler]

Hi -

After a brief consultation with kenistoj last night, it seems
appropriate to merge the user-space probing work to mainline.  It is
by no means finished (several serious bugs are being worked on), but
it now causes only one feature regression: I have had to disable the
symbol-table-only probing logic.  Jim and I will restore that before
too long.

- FChE