include: Use printf(1) instead of shell loops for header symlinks

[arichardson]

Summary

Using a shell for loop means we have to spawn a separate install(1)
process for each header that is symlinked. This patch uses printf(1) to
generate an argument list that has been prefixed with the correct number
of ../ and then uses a single install(1) invocation.

This reduces the number of execve() calls during make includes from 2442
(with D27622) to 1382. Running make symlinks in include/ now spawns 214
processes instead of 1276 without this patch.

Test Plan

find ${WORLDTMP}/usr/include -xtype l only prints sys/nvpair.h but that was already broken before due to the openenzfs typo introduced in rS364746. I'll commit a fix for the typo once git is writable.

[jhb]

printf(1) reusing the format string is a cute trick, I hadn't realized it did that.

[arichardson]

rebase after latest commits