leading whitespace in the value of var (because strtoimax() does)
but did not allow trailing whitespace. The effect is that some
cases where $(( ${var:-0} )) would work do not work without the $
expansion.
Fix that - allow trailing whitespace. However, continue to insist
upon at least one digit (a non-null var that contains nothing but
whitespace is still an error).
Note: posix is not helpful here, it simply requires that the variable
contain "a value that forms a valid integer constant" (with an optional
+ or - sign).
e76b58b2db