Make fish install a command_not_found handler in non-interactive uses.

Previously, fish's command_not_found handler would be installed in
__fish_config_interactive. Errors that occured early in startup (e.g. in
config.fish) or in non-interactive mode would therefore not be reported.
With this change, fish now exposes its default cnf handler as
__fish_default_command_not_found_handler . config.fish then installs a
cnfh that invokes the default. When fish goes interactive, the initial
cnfh is overwritten with a fancier one, that may in turn fall back to
invoking the default.
Fix to stop reporting config.fish execution as coming from "standard
input" within backtraces