src/chfn.c: Partially revert "lib/, src/: Use strsep(3) instead of its pattern"

src/chfn.c: Partially revert "lib/, src/: Use strsep(3) instead of its pattern"

This partially reverts commit 16cb664865541162c504a6f5ef5ca4b38b5e0c9a.

I'll try to reintroduce this change more carefully.
For now, let's revert to a known-good state.

The problem was due to accidentally ignoring the effects of the 'break'
on the 'cp' variable.

Fixes: 16cb66486554 (2024-07-01; "lib/, src/: Use strsep(3) instead of its pattern")
Closes: <https://github.com/shadow-maint/shadow/issues/1210>
Link: <https://github.com/shadow-maint/shadow/pull/1213>
Link: <https://github.com/shadow-maint/shadow/pull/1212>
Reported-by: Chris Hofstaedtler <zeha@debian.org>
Suggested-by: Chris Hofstaedtler <zeha@debian.org>
Tested-by: Chris Hofstaedtler <zeha@debian.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>