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

doc/TODO

@@ -11,7 +11,6 @@ Packages:
    in a package, to know when update it.
  * Create /etc/issue.
  * Create files in /etc/skel for new users.
- * Register shells in /etc/shells.
  
 xbps-bin:
  * Add support to handle conf_files and keep_dirs from package metadata.