Adds a `strverscmp(3)` function to libc, a GNU extension I implemented by reading its glibc manual page.
It orders strings following a much more natural ordering (e.g. "ent1 < ent2 < ent10" as opposed to "ent1 < ent10 < ent2" with `strcmp(3)`'s lexicographic ordering).
Also adds `versionsort(3)` for use as `scandir(3)`'s `compar` argument.
Updates manual page for `scandir(3)` and adds one for `strverscmp(3)`.
Adds tests for `strverscmp(3)`.
Original diff: D35783