C library: Add ffs(). Add strings.h. Move strcasecmp, strncasecmp, bzero,...

C library:  Add ffs().  Add strings.h.  Move strcasecmp, strncasecmp, bzero, bcmp, and bcopy to where they belong in strings.h.h, not string.h.  bzero, bcmp, and bcopy are legacy functions; the contemporary counterparts should be used instead.