Default math scale to 6

This changes the behavior of builtin math to floating point by default.
If the result of a computation is an integer, then it will be printed as an
integer; otherwise it will be printed as a floating point decimal with up to
'scale' digits past the decimal point (default is 6, matching printf).
Trailing zeros are trimmed. Values are rounded following printf semantics.

Fixes #4478

tests/math.in

@@ -2,6 +2,7 @@ logmsg Validate basic expressions
 math 3 / 2
 math 10/6
 math -s0 10 / 6
+math 'floor(10 / 6)'
 math -s3 10/6
 math '10 % 6'
 math -s0 '10 % 6'
@@ -14,6 +15,19 @@ math 5 \* -2
 math -- -4 / 2
 math -- '-4 * 2'
 
+logmsg Validate some integral computations
+math 1
+math 10
+math 100
+math 1000
+math '10^15'
+math '-10^14'
+math '-10^15'
+
+math -s0 '1.0 / 2.0'
+math -s0 '3.0 / 2.0'
+math -s0 '10^15 / 2.0'
+
 logmsg Validate how variables in an expression are handled
 math $x + 1
 set x 1
@@ -21,7 +35,7 @@ math $x + 1
 set x 3
 set y 1.5
 math "-$x * $y"
-math -s1 "-$x * $y"
+math -s0 "-$x * $y"
 
 logmsg Validate math error reporting
 not math '2 - '
@@ -31,5 +45,5 @@ not math 'sin()'
 not math '2 + 2 4'
 not math
 not math -s 12
-not math 2^650
+not math 2^999999
 not math 1 / 0