top: Add unobtrusive XDG support

By default the file HOME/.toprc will be prefered.  This ensures there
should be minimal breakage even if this file is later created by some
other means.  Otherwise we will follow the new behaviour described by
the XDG Base Directory Specification:

If the XDG_CONFIG_HOME environment variable is available we will attempt
to use this as XDG_CONFIG_HOME/procps/toprc otherwise we will fall-back
to HOME/.config/procps/toprc instead.

Signed-off-by: Earnestly <zibeon@gmail.com>