20260119
  Fix an obscure bug with interactive use:  a syntax error could cause
the current problem name to be lost.  For example, if foo contains

	var x; var y >= 0;
	minimize o: x^2 + (y - 10)^2;
	minimize f: (x - y)^2;
	problem Outer: x, o;
	problem Inner: y, f;
	solve outer; # not defined

then (the interactive) "include foo" gives error message

	foo, line 6 (offset 146):
		outer is not defined

after which "show Inner;" gave "Inner is undefined. ..."
instead of showing Inner.

  New builtin functions
	logistic(x) = 1 / (1 + exp(-x))
and
	signpow(x,y) = if x < 0 then -(-x)^y else x^y = sign(x)*|x|^y.

  Allow builtin functions to be replaced by imported functions,
so, e.g., if amplfunc.dll contains logistic and signpow as above, then

	function logistic;
	function signpow;
	print logistic(1), signpow(-2,3);

works (before or after the latest changes), giving output

	0.7310585786300049 -8

the same out as just

	print logistic(1), signpow(-2,3);

after the current changes.  Previously, without the function,
declarations, the above line gave error message

	syntax error
	context:  print logistic( >>> 1) <<< , signpow(-2,3);

Use of logistic and signpow by a solver requires either a suitable
amplfunc.dll or an updated (relinked) solver.