svr4, again

[Maxime Villard <max%m00nbsd.net@localhost>]

Should I re-start a fight about dropping COMPAT_SVR4? Because I keep finding
bugs, and it's becoming tiring. Over time I've come across at least a good
dozen of bugs in it, by just grepping through the tree searching for unrelated
things. The last one is this [1], while working on kcov, exit1() is called but
the proc mutex is not held.

The thing is just broken beyond repair.

It is dead wood that consumes APIs and makes stuff harder to change, just like
the network components we (thankfully) retired.

[1] https://nxr.netbsd.org/xref/src/sys/compat/svr4_32/svr4_32_signal.c#661