Add $register_shell var, to (un)register shells at post-{inst,rm} for binpkgs.

Multiple shells can be (un)registered by a single package, like:

register_shell="/bin/zsh-foo /bin/zsh-blah"

--HG--
extra : convert_revision : 0172a74b41e26089da97fcdc8cc2d5cb6ae147f2

templates/bash/template

@@ -22,6 +22,7 @@ long_desc="
  modification."
 
 base_chroot=yes
+register_shell="/bin/bash"
 
 Add_dependency run	glibc
 Add_dependency full	ncurses