Fix broken cider-eval-ns-form

[danskarda]

This pull request fixes bugs in cider-eval-ns-form introduced by recent addition of undef-all parameter:

1. There was a typo in interactive parameter. "p" is a number which evaluates to true even if it is 0. Parameter is used in when condition, so undef-all branch is always executed.

2. It uses magic value of (match-string 0). For me it returns string (ns and undef fails. Probably regexp in clojure-find-ns was recently changed, which broke cider-eval-ns-form without anybody noticing. I do not think it is a good idea to rely on magic side-effects of previous functions, so I replaced match-string with return value of cider-find-ns.

There is a last issue with undef in general. It undefs symbols of classes which are imported by default (eg Throwable). But I guess this is more an issue of nrepl middleware.

[vemv]

LGTM! Thanks for the explanation, code makes sense to me as well.

Please update the changelog as surely we're good to go.

[danskarda]

Changelog updated.

[yuhan0]

Sorry about introducing so many bugs! I've just checked the other functions affected by that PR and they seem to be fine, it's just cider-eval-ns-form which escaped my attention because I've never used it.

The last point about symbols like Throwable being undef'ed was brought up in #3194, unfortunately it requires a update to cider-nrepl version after the fix is merged.

[bbatsov]

No worries! @danskarda Thanks for fixing this!

[bbatsov]

(I'll likely cut CIDER 1.4.1 after we merge the undef-all cider-nrepl update)

[vemv]

Done https://github.com/clojure-emacs/cider/blob/v1.4.1/CHANGELOG.md#141-2022-05-25