Move WB_AUTH into session keyring on Linux

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Currently the console secret is an environment variable that is inherited by process children. On Linux, the kernel's key management facility can store the secret so it can not be read through procfs. On FreeBSD, Solaris, etc., this will remain the same. To implement this change, all -- currently 5 -- instances of WB_AUTH in the library and application (https://github.com/WhiteBeamSec/WhiteBeam/search?q=WB_AUTH) need to be updated to use a platform-specific function. On Linux, this is add_key and request_key.

https://man7.org/linux/man-pages/man2/add_key.2.html
https://man7.org/linux/man-pages/man2/request_key.2.html
https://man7.org/linux/man-pages/man7/session-keyring.7.html

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This would also eliminate needing to spawn a shell with the whitebeam --auth command, which must also be revised to read from stdin (for automation).