Skip re-setup when creating a child session

[nunosilva800]

When binding.irb is called, the new session is setup without command line arguments:

irb/lib/irb.rb

Lines 1516 to 1517 in a641746

	# Setup IRB with the current file's path and no command line arguments
	IRB.setup(source_location[0], argv: [])

This means that the child session does not respect the parent command line arguments.
Therefore when using $ irb -f (to ignore RC file), and opening a child session with binding.irb, the new session will load the RC file (if it exists).

See more in:

• IRBRC file is loaded every time ruby reaches binding.irb #625

This solution is different from:

• Respect parent command line arguments in nested IRB scopes #848

1. It does not call IRB.setup when creating a new IRB instance, unless the current session was never initialized
2. Adds IRB.initialized? to know if the conf init was called at least once

Closes #625

[st0012]

This is amazing, thank you!