libc-package.bbclass: Obey base_bindir for shell shebang.
package.bbclass: Obey base_bindir/base_sbindir (incl. shell shebang).
update-alternatives.bbclass: Obey base_bindir for shell shebang.
update-rc.d.bbclass: Obey base_bindir for shell shebang.
useradd.bbclass: Obey base_bindir for shell shebang.
shadow: Obey sysconfdir.
update-rc.d: Rewrite installed script to obey sysconfdir.