Make `fish_indent`, `fish_key_reader` and `fish` recognize and assign
the same meaning to the `-d` and `-D` flags. Also, fix some errors and
stylistic issues in the associated man pages.
Fixes#3191
The original `key_reader` program was useful but didn't do much that `xxd`
or `od -tx1z` didn't do. Furthermore, it wasn't built and installed by
default. This change adds features that make it superior to those programs
for decoding interactive key presses and makes it a first-class citizen
like the `fish_indent` program that is always available.
Fixes#2991
