Rather than killing the process with close, read EOF after sending the
"exit" command and wait for OS cleanup (per the expect examples).
Not cleaning up with wait caused expect to crash on all 32-bit platforms
including i586 and armv7l with "alloc: invalid block: 0xbf993ccb: 3d 3b".
64-bit platforms were not affected, for reasons that are not clear.
Don't import the bash history if the user has specified that a non-default
fish history file should be used. Also, rename the var that specifies
the fish history session ID from `FISH_HISTFILE` to `FISH_HISTORY`.
Fixes#4172
Using the FISH_HISTFILE variable will let people customise the session
to use for the history file. The resulting history file is:
`$XDG_DATA_HOME/fish/name_history`
Where `name` is the name of the session. The default value is `fish`
which results in the current history file.
If it's set to an empty string, the history will not be stored to a
file.
Fixes#102
