Synchronize <strings.h> with latest FreeBSD

Synchronize <strings.h> with latest FreeBSD

Include <strings.h> in <string.h> if __BSD_VISIBLE like on FreeBSD.
Remove redundant declarations from <string.h>.  Make ffsl(), ffsll(),
strncasecmp(), strcasecmp_l(), and strncasecmp_l() visible via
__BSD_VISIBLE instead of __GNU_VISIBLE.  Add fls(), flsl(), and flsll()
to <strings.h> if __BSD_VISIBLE.

Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>