Free42 SOLVE: Better sign reversal detection

[Thomas Okken]

The reason this is tricky is that you don't want to show Sign Reversal for functions that never return exactly 0 because of finite precision, without actually being discontinuous, like

00 { 22-Byte Prgm }
01▸LBL "LIN"
02 MVAR "X"
03 RCL "X"
04 2
05 ×
06 3
07 ENTER
08 1/X
09 ×
10 -
11 END