Rewrite cpnumber() to fix `%2(msg)' with 123 overflowing to `?23'.

Rewrite cpnumber() to fix `%2(msg)' with 123 overflowing to `?23'.

Reported in 1.7 by Ken on nmh-workers yesterday.
`It seems this was introduced in commit 92128dacf'.
We now use the C library to format the number, and then manipulate the
result.  A temporary charstring_t is no longer used.
Also adds left justification using negative width.