tftp-proxy: move to the pf package

tftp-proxy is part of pf and isn't useful without it, so move it to the
pf package (where its friend ftp-proxy already lives).

Reviewed by:	manu, kp, des, emaste
Approved by:	des (mentor)
Differential Revision:	https://reviews.freebsd.org/D50145

libexec/tftp-proxy/Makefile

@@ -1,5 +1,6 @@
 .PATH:	${SRCTOP}/contrib/pf/tftp-proxy
 
+PACKAGE= pf
 PROG=	tftp-proxy
 SRCS=	tftp-proxy.c filter.c
 MAN=	tftp-proxy.8