MFC r311459:

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
Differential Revision: https://reviews.freebsd.org/D9052

MFC r311461:

Also remove unnecessary extern keywords from tcpd.h.

Noticed by: kib

MFC r311556:

After r311459, some ports can break, because a few of the newly added
prototypes in <tcpd.h> use FILE. Pull in a minimal forward declaration
of FILE from <stdio.h> to minimize impact. Sorry for the breakage.

Reported by: Shawn Webb <shawn.webb@hardenedbsd.org>