multiprocessing: allow Spack to run uninterrupted in background (#14682)

Spack currently cannot run as a background process uninterrupted because some of the logging functions used in the install method (especially to create the dynamic verbosity toggle with the v key) cause the OS to issue a SIGTTOU to Spack when it's backgrounded.

This PR puts the necessary gatekeeping in place so that Spack doesn't do anything that will cause a signal to stop the process when operating as a background process.