Disable background evals when process init failed

[lionel-]

These changes are aimed to prevent background eval hangs when ESSR failed to load.

• Add a bg-eval-disabled process property. It's set to t when ESSR couldn't be loaded.
• Add a (ess-can-eval-in-background) defun that checks for the global variable as well as the process property. Now used throughout ESS instead of just checking for the global var.
• Add a (ess-get-next-available-bg-process) defun to get a process for which bg evals are allowed. Used in completion helpers.

[lionel-]

Merging now but comments are welcome.