Set DEFAULT_INIT_SHELL to /sbin/openrc-run for Gentoo and Alpine

Gentoo and Alpine always set the rc init scripts' shebang to
#!/sbin/openrc-run, whether or not openrc is installed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Peter Levine <plevine457@gmail.com>
Closes #12683
Closes #12692