pf: clean up pflow sockets on jail removal

[kp]

Summary

pflow opens sockets in the kernel to transmit netflow information.
If this is done in a (vnet) jail these sockets end up preventing the removal of
the jail. The VNET_SYSUNINIT() vnet_pflowdetach() function doesn't get called,
but that's the function that would remove the sockets.

Install a callback on the PR_METHOD_REMOVE jail callback and close the sockets
there. This ensures that the jail can get cleaned up.

Sponsored by: Rubicon Communications, LLC ("Netgate")

sys/netpfil/pf/pflow.c

 # Autogenerated - do NOT edit!
 
 DIRDEPS = \
         include \
         include/arpa \
         include/xlocale \
         lib/clang/libclang \
         lib/clang/libllvm \
         lib/libc++ \
         lib/libedit \
         lib/libexecinfo \
+        lib/libkvm \
+        lib/liblzma \
+        lib/libz \
         lib/msun \
-        lib/ncurses/ncursesw \
-        lib/ncurses/panelw \
+        lib/ncurses/panel \
+        lib/ncurses/tinfo \
+        usr.bin/clang/lldb-tblgen.host \
 
 
 .include <dirdeps.mk>
 
 .if ${DEP_RELDIR} == ${_DEP_RELDIR}
 # local dependencies - needed for -jN in clean tree
 .endif