unable to abbreviate datafile subcommand for set/show command because of overlapping with data subcommand

[B̅]

Currently (commit a8a8a8) the abbreviation of datafile subcommand for set/show commands is "data" (ref).
Since it overlaps with the string "data" of data subcommand (ref), datafile subcommand is unable to be abbreviated (gnuplot utility fails with "unrecognized option").

I suggest defining a string "df" or "datf" as the abbreviation of datafile subcommand.

-{ "data$file", S_DATAFILE },
+{ "datafile", S_DATAFILE },
+{ "df", S_DATAFILE },