net/boinc-client: configure alternate platform according to ${ARCH}, fix options

Improve port:

• fix options - dependencies, plist, configure options;
• fix check for symlinks in pkg-deinstall;
• fix warnings from portclippy;
• configure alternate platform according to ${ARCH} for option LINUX

(patch from Älven <alster@vinterdalen.se>).

PR: 272727
Approved by: ler (maintainer, timeout > 7 months)