bug#23179: 25.0.92; Restore `M-,' to continue etags search

[Juri Linkov]

>> We should welcome patches to provide implementation of tags-search
>> based on Xref machinery.  Would you or someone else like to work on
>> that?
>
> Y'all can try 'M-x project-find-regexp' or 'M-x project-search' in the
> meantime, depending on the personal preference for the UI.

I tried 'M-x project-find-regexp' and 'M-x project-search' and found
the following problems:

  M-x project-search

    1. can't use M-n to get the default value such as a word under point
       like project-find-regexp does

  M-x fileloop-continue

    2. its name prefix is inconsistent with the prefix of its
       counterpart command project-search

    3. no keybinding

  M-x project-find-regexp

    4. shouldn't it support an optional more compact grep-like output
       layout as well (where file names are on the same line with matches)?