Backport path_find_first_component()

Backport path_find_first_component()

This is a prerequisite to backport path_startswith_full().

path_find_first_component() was introduced in systemd v249, in commit
0ee54dd4e2f: "path-util: introduce path_find_first_component()".

The function didn't change since it was introduced: it's still the same code in
systemd v260. However the test associated with it was slightly adjusted
afterward.

This commit backports the fonction and associated unit tests from systemd v260
(ie. with the latest version of the unit tests).

Forwarded: not-needed

Gbp-Pq: Name CVE-2026-29111-1.patch