Ignore read-only variables

[kouk]

These variables cannot be set in fish:

• PWD
• SHLVL
• history
• pipestatus
• status
• version
• FISH_VERSION
• fish_pid
• hostname
• _
• fish_private_mode

If a script exports this variable an error message bass should not try and set them in fish because this error message will be emitted:

set: Tried to change the read-only variable “_”

In this case it's particularly annoying becausethe underscore variable is automatically set by bash:

Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command.

[edc]

Thanks for the fix!