#4860 simplim%acos: acos(±inf) returns und rather than infinity

[David Scherfgen]

Spotted by Claude:

limit(acos(x), x, minf);   /* actual: und;  expected: infinity */
limit(acos(x), x, inf);    /* actual: und;  expected: infinity */

Fix: Copy behavior of asin.

[David Scherfgen]

Fixed by commit [8c36f9].