assistant: Fix a crash on startup by avoiding using forkProcess

assistant: Fix a crash on startup by avoiding using forkProcess

ghc 8.8.4 seems to have changed something that broke code that has been
successfully using forkProcess since 2012. Likely a change to GC internals.

Since forkProcess has never had clear documentation about how to
use it safely, avoid using it at all. Instead, when git-annex needs to
daemonize itself, re-run the git-annex command, in a new process group
and session.

This commit was sponsored by Luke Shumaker on Patreon.