sci-misc/boinc: Migrated boinc.init to use start-stop-daemon

sci-misc/boinc: Migrated boinc.init to use start-stop-daemon

The boinc init script can no longer start, attach or stop the boinc
manager if openrc-0.20 and newer is used, because openrc no longer
passes $SHELL to init scripts.

To solve the issue the init script has been migrated to use
start-stop-daemon instead of using 'su' directly.

This change should have no impact for users of previous openrc
versions.

Gentoo-Bug: https://bugs.gentoo.org/574260
Package-Manager: portage-2.2.28
Closes: https://github.com/gentoo/gentoo/pull/1244

Signed-off-by: Patrice Clement <monsieurp@gentoo.org>