ffs.3: Add a STANDARDS section to the manual page

The ffs() function conforms to IEEE Std 1003.1-2008 ("POSIX.1").
The ffsl() and ffsll() functions conform to IEEE Std 1003.1-2024 ("POSIX.1").

Reviewed by: ziaee
MFC after: 5 days
Differential Revision: https://reviews.freebsd.org/D53352