Use the correct size when allocating the cmdbuf string.

Use the correct size when allocating the cmdbuf string.

cmdlengthdelta is the size of the header and we were using it to
allocate a buffer to store the command line. This would mean that
the cmdbuf could be too short. In practice this was never noticed unless
you usually run top -a. On a stock FreeBSD system you can see the
problem by running sendmail and then running top -a on a big terminal
window. In practice this doubles to size available to cmdbuf since the
header is around 65-68 bytes.

Reviewed by:	adrian