lint: warn about extern declarations outside headers

https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

If a library needs a symbol from another library, pull that library in
explicitly, even if the DT_NEEDED closure would normally already ensure
the presence.

These directories default to WARNS?=5

Add sources, Makefiles, and manual pages for ppath(3), a
property-container path library that is usable from both userland and
the kernel.

TBD: hook this into the build, update set lists, and add au

Add sources, Makefiles, and manual pages for ppath(3), a
property-container path library that is usable from both userland and
the kernel.

TBD: hook this into the build, update set lists, and add automated
tests.

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