Consistent naming scheme for cairo-based terminals

[Ethan A M. <merritt@u.washington.edu>]

I have been playing with the cairopng terminal (see 
<http://skuld.bmsc.washington.edu/people/merritt/gnuplot/demo_cairopng/> 
and of course we have the cairopdf terminal already in cvs.

Both work OK whether or not the corresponding alternative libgd and pdflib 
terminals are also present.  If all four are present then it doesn't seem 
so bad to have to type "pdf", "png", "cairopdf", and "cairopng", although 
it is somewhat annoying to have to type 7 characters to distinguish 
cairopdf from cairopng.

But if you build *only* the cairo-based terminals, then the naming scheme 
seems counterproductive.  People will expect "set term png" to work, but 
instead they must type "set term cairopng".

So I propose that we rename the two cairo based terminals, and any future 
ones, to use instead the names
	set term pdfcairo
	set term pngcairo
and so on.  This has the advantage that if no separate "png" driver is 
present, then "set term png" is unambiguous and will select the pngcairo 
driver instead.  If the libgd-based png driver *is* present, then you must 
type at "set term pngc" to select the cairo-based alternative, but that's 
still shorter than having to type 7 or more characters as you must do now.

This requires changing:
 - the name of the terminal in the corresponding TERM_TABLE entry  
 - the name of the terminal in the docs
 - the test in term.c (change_term) so that it is legal to have one
   terminal name be a leading substring of another.

Proposed patch uploaded to SourceForge.

If would also be nice to merge the file .../term/cairopng.trm into 
cairo.trm so that the code can be shared.  If for some reason that is not 
possible, then it would be logical to rename cairo.trm to cairopdf.trm.
But the patch does neither of these things.

Any objections or better suggestions?

	Ethan