Support the 'spawn' method of starting processes

Add support for the spawn method of starting processes.

• .gitlab-ci.yml: Add a test with spawn enabled
• _frontend/cli: support 'BST_FORCE_START_METHOD' env var
• Support pickling jobs if the platform requires it
• Abstract mp Queue usage, prep to spawn processes
• utils: uppercase global _MAIN_PID, move to top

This is vital for supporting the win32 platform: #1021