rename TERM_XMIN to GPVAL_TERM_XMIN

[Petr M. <mi...@ph...>]

Recently, new variables
	TERM_XMIN TERM_XMAX TERM_YMIN TERM_YMAX
have been added. I propose to rename them to 
	GPVAL_TERM_XMIN GPVAL_TERM_XMAX GPVAL_TERM_YMIN GPVAL_TERM_YMAX
so that 

- they have same name as all other autogenerated variables (see 
  "show variables all"), like GPVAL_TERM, GPVAL_TERMOPTIONS, GPVAL_X_MIN, 
  etc.

- they are "read-only", try e.g.

gnuplot> GPVAL_X_MIN=99
         ^
         Cannot set internal variables GPVAL_ and MOUSE_

---
PM