du: document the TIME_STYLE env variable in usage

du: document the TIME_STYLE env variable in usage

The impact of the TIME_STYLE environment variable on du(1) was only
documented in the Texinfo manual.  To avoid surprises for users,
also mention TIME_STYLE in the usage text, i.e., for --help and man.
Organize similar as in ls(1), but as du(1) has slightly different
behavior it would be hard to share the translation.

* src/du.c (usage): Shorten the description of --time-style, and refer
to an additional --time-style / TIME_STYLE description below.