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.

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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.