Log Message:
mk/misc: in show-all, list values of *_ENV and *_ARGS in separate lines

The *_ENV and *_ARG values are typically very long, and reading them in
a single line is unnecessarily difficult. Therefore, each of their
values is listed on a separate line, for example:

fetch:
  usr   DIST_PATH (undefined)
  pkg   MASTER_SITES = \
                http://ftp.gnome.org/pub/GNOME/sources/glib/2.56/ \
                ftp://ftp.gnome.org/pub/GNOME/sources/glib/2.56/ \
                ftp://ftp.cse.buffalo.edu/pub/Gnome/sources/glib/2.56/ \
                https://download.gnome.org/sources/glib/2.56/ \
                # end of MASTER_SITES
  pkg   DIST_SUBDIR (undefined)