sci-misc/boinc: Fix bashisms in init script and enhance OpenCL support

sci-misc/boinc: Fix bashisms in init script and enhance OpenCL support

This commit fixes the following issues:

 - https://bugs.gentoo.org/show_bug.cgi?id=620818
   The previous init script consisted of many bashisms that have been removed.

 - BOINC supports OpenCL unconditionally. This commit adds an informational
   text about its support to the ebuilds.

 - For OpenCL to work, the currently used libOpenCL.so must be symlinked into
   the BOINC working directory, just like libcudart.so. The init script has been
   updated to take care of symlinking libOpenCL.so.

Closes: https://github.com/gentoo/gentoo/pull/4887
Package-Manager: Portage-2.3.6, Repoman-2.3.2