Move fts.h and functions from libroot to libbsd.

These are BSD extensions, not POSIX functions. They were needed
in libroot by the previous versions of the ftw/nftw implementations,
but the musl versions do not need them, and so we can move them to
libbsd.

This is a minor ABI break, but hopefully whatever was using them
in libroot also links to libbsd. If not, that's an easy enough fix.
(These were only added to libroot in 2013.)