expose built-in hotkey functions to the command line

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

After the addition of new automatic GPVAL_ variables, I wish to be able to 
rebound some hotkeys, for example:

bind Right "if (GPVAL_SPLOT) builtin_rotate_right; \
	else set xrange [GPVAL_X_MIN+100:GPVAL_X_MAX+100]; reread"

I propose to move the "builtin_" functions from the default "bind" command 
among other functions callable from the command line. I guess there is a 
very small probability that users would call functions by these names and 
thus it would break some scripts.

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PM