Presently, `isatty` only works on a handful of keywords. Here it
is rewritten to be able to take any path, device or fd number as
an argument, and eliminates errors printed to stdout.
Per discussion in #1228, using `builtin test -c` within a pipe to
test special file descriptors is not viable, so this implementation
specifcially uses `command test`. Additionally, a note has been
added to the documentation of `test` regarding this potential
aberration from the expected output of the test utility under the
'Standards' section.
Large list of changes, including formatting and typos for most commands.
More substantive changes have been made to alias, bind, block, break,
builtin, case, cd, commandline, count, else, emit, fish_config, funced,
function, functions, history, math, mimedb, nextd, not, popd, prevd,
pushd, pwd, random, read, set, set_color, switch, test, trap, type,
ulimit, umask, and while.
