expr: TIP 526, only support a single arg

Avoid unexpected issues by concatenating multiple arguments.
This does create an incompatibility with early versions, but
it is generally trivial to convert existing code to one of two forms:

1. expr {$a + $b}    -- usually correct
2. expr "$a + $b"    -- usually incorrect

Signed-off-by: Steve Bennett <steveb@workware.net.au>