The previous change to use `argparse` for parity with every other
builtin and function introduced a regression. Invocations that start
with a negative number can fail because the negative value looks like an
invalid flag.
This would fail on very long numbers, e.g.
`math "1 + 1233242342353453463458972349873489273984873289472914712894791824712941"`
would now return "42", where it previously returned the correct "1233242342353453463458972349873489273984873289472914712894791824712942".
This reverts commit 26e781ef5a.
It's not the case that macOS and old BC doesn't respect this environment
variable, just that they don't have special behavior when it's set to 0.
However, there is rather universal favorable behavior with a value of 2.
Output is of the form:
\
999999999999999999999999999999999...
with the second line being arbitrarily long. So just grab that line
instead of stitching with `string`.
This can yield a 25-30% speedup.
This makes it easy for the user to request floating point output with the
desired number of digits after the decimal point (not to be confused with
significant digits).
Note that this is just a thin wrapper so someone can say `math -s3 10 / 3`
rather than `math "scale=3; 10 /3"`.
Resolves#1643
