Emply contemporary function prototypes in bootpd, rather than relying on

Emply contemporary function prototypes in bootpd, rather than relying on
locally defined K&R prototypes in .c files; use appropriate casts for
pointer types now that types for arguments are available at compile time.
This ensures that compilers with multiple incompatible calling conventions
can select the correct calling convention for external functions.

Sponsored by:	DARPA, AFRL
MFC after:	1 week