Support XDG base directory for gnuplotrc

Dear list,

In merge request #12 I propose the addition of the XDG base directory
specification for configuration file paths.

    https://sourceforge.net/p/gnuplot/gnuplot-main/merge-requests/12/

There is already some discussion on the ticket.  It seems that the
overall opinion is in favor of this addition.  Is this correct?  If that
is the case, could you please review the code?  After that I will write
some words in the docs and we should be good to go.

Cheers, Henri