Make new sd-path configuration search functionality generic (#39684)

Reverts systemd/systemd#38680

After taking a closer look I'm not convinced by the approach, see below.

First of all, all other SD_PATH_SEARCH_* are either somewhat generic,
i.e. encode the common prefix for configurations, binaries, etc., or are
subdirectories under systemd/ hence in our own "domain". The
tmpfiles/sysctl/binfmt we don't prefix with "systemd" precisely because
the concept is generic and there're actually other impls of them. A
specific SD_PATH_SEARCH_SYSCTL doesn't fit into our existing scheme.
Instead something along the lines of "SEARCH_SYSTEM_CONFIGURATION" shall
be introduced, and consumers will just suffix
sysctl.d/tmpfiles.d/binfmt.d for the final result.

And secondly, I don't grok why systemd-sysctl now unnecessarily calls
into sd-path to obtain the fixed search path. None of our other tools do
that.

-----------

An alternate approach, SD_PATH_SYSTEM_SEARCH_CONFIGURATION, which does
exactly above, will be introduced instead. It provides a universal
interface for querying any system config with our idiomatic
/etc/:/run/:/usr/local/lib/:/usr/lib/ hierarchy.

test/units/TEST-74-AUX-UTILS.path.sh

@@ -51,6 +51,8 @@ assert_eq "$(systemd-path --suffix='' system-runtime)" "/run"
 assert_eq "$(systemd-path --suffix='🤔' system-runtime)" "/run/🤔"
 assert_eq "$(systemd-path --suffix=hello system-runtime)" "/run/hello"
 
+assert_eq "$(systemd-path system-search-configuration --suffix=sysctl.d)" "/etc/sysctl.d:/run/sysctl.d:/usr/local/lib/sysctl.d:/usr/lib/sysctl.d"
+
 # Note for the stuff below: everything defaults to $HOME, only the desktop
 # directory defaults to $HOME/Desktop.
 #