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 */ ^
Add prototypes using __P, since that is what was done in other
tcpwrapper headers.
While here, garbage collect the incompatible rfc931() function from
scaffold.c, as it is never used.