Replace su-exec with gosu

There's a major issue with `su-exec` whose fix has gone unreleased for 5 years (typos leading to running code as root, the opposite of the purpose of the program).

This also decreases our Debian vs Alpine variance.

Due to user scripts/downstream code potentially using `su-exec`, I have included a compatibility symlink to `su-exec` for all versions less than the 17 pre-release.

Author: tianon