Log Message:
mksh: Don't set PKG_SHELL when used for bootstrap.

This can cause circular problems where the +INSTALL script requires the shell
itself, but also avoids issues trying to add to /etc/shells during bootstrap
where it may not be appropriate, or done outside of the context of where the
bootstrap tarball will finally be installed.

Users who use mksh as a bootstrap shell can simply add to /etc/shells manually
after installing or extracting the bootstrap as required.

While here remove unnecessary extra indents.