+SHELL.
* Turn PKG_REGISTER_SHELLS into a variable that can be set in the shell
environment so that admins can make a choice when installing from
binary packages.
* PKG_SHELL is now a list of paths, and if the path is relative, then it
is taken to be relative to ${PREFIX}. Convert packages that set
PKG_SHELL to take advantage of this new feature by changing the full
paths to the shells into relative paths.