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.
