Introduce new syntax for com_ls().

Introduce new syntax for com_ls().

The ls command is the only command that takes one-letter arguments
which immediately follow the option. For example, -lv activates
verbose listing mode. If we ever want to support short option combining
(like in rm -rf) for paraslash commands, the one-letter arguments are
problematic because, in the above example, -lv can also be interpreted
as -l -v.

This commit encourages a different syntax for the -l and -s options
of the ls command while keeping the one-letter options working
for backwards compatibility. The new official way to enable verbose
listing mode is -l=v. This is analogous to how the "stat" and "touch"
commands expect their arguments.

The ls completer and the documentation are also updated according to
the new syntax.