`find-tag-regexp` doesn't find all the matches

[Raffaele Ricciardi <rfflrccrd@gmail.com>]

When I use `find-tag` to lookup a symbol, I get more matches than with
`find-tag-regexp`.

For example, to lookup "sscanf" with `find-tag`, I type `M-. sscanf RET`
and then `C-u M-.` repeatedly, and I get this matches, one after
another:

     # define sscanf __isoc99_sscanf
     sscanf,
     extern int __REDIRECT_NTH (sscanf, (const char *__restrict __s,

Then `C-u M-.` starts to find matches where "sscanf" is a substring,
therefore I stop.

If instead I type `C-M-. \_<sscanf\_> RET`, I get this match:

     sscanf,

Pressing `C-u C-M-.` again sometimes finds the other matches, and
sometimes says "No more tags matching \_<sscanf\_>".  However,
`M-. sscanf RET` followed by `C-u M-.` still works.  It seems that
`find-tag-regexp` gets stuck, whilst `find-tag` doesn't.  Maybe the
issue is that there are two tag tables in `tags-table-list` (I am
guessing because I haven't been able to reproduce this issue with one
table only).

Thanks for you help.