Put proper prototypes in tcpd.h

Clang 4.0.0 complains about tcpd.h's not-really-prototypes, e.g.:

/usr/include/tcpd.h:75:24: error: this function declaration is not a prototype [-Werror,-Wstrict-prototypes]
extern int hosts_access();              /* access control */
                       ^

To fix this, turn these declarations into real prototypes. While here,
garbage collect the incompatible rfc931() function from scaffold.c, as
it is never used.

Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D9052