libc: Rename fscandir{,_b}() to fdscandir{,_b}().

[des]

Summary

This seems to fit the pattern better (e.g. fdopendir()).

I've added weak references to ease the transition, but since it's only
been a few days, we can remove them (and the ObsoleteFiles entries for
the manual pages) before we branch stable/15.

Fixes: deeebfdecab5
Sponsored by: Klara, Inc.