This is the first step to implementing issue #4200 is to stop subclassing
env_var_t from wcstring. Not too surprisingly doing this identified
several places that were incorrectly treating env_var_t and wcstring as
interchangeable types. I'm not talking about those places that passed
an env_var_t instance to a function that takes a wcstring. I'm talking
about doing things like assigning the former to the latter type, relying
on the implicit conversion, and thus losing information.
We also rename `env_get_string()` to `env_get()` for symmetry with
`env_set()` and to make it clear the function does not return a string.
This takes a string that is then split upon like `string split`.
Unlike $IFS, the string is used as one piece, not a set of characters.
There is still a fallback to IFS if no delimiter is given, that
behaves exactly as before.
Fixes#4156.
The `read` command `-m` and `--mode-name` vars are now deprecated and do
nothing other than result in a warning message. The `read` command now
honors the `FISH_HISTORY` var that is used to control where commands are
read from and written to. You can set that var to the empty string to
suppress the use of both history files. Or you can set it to a history
session ID in which case that will limit the `read` history that is
available.
Fixes#1504
Running the tests on travis revealed that some compilers (or at least
with some options) call the wrong struct constructor if there is more
than one struct with the same name but differing definitions.
