Added %I and %O formatters to varnishncsa

This adds %I and %O formatters to varnishncsa, which are total bytes
received and total bytes sent. These are similar to the same
formatters in apache (with mod_logio). Useful for bandwidth accounting.

Patch by: David Robertson
Document %I and %O ncsa format specifiers
Only parse ReqAcct once
Keep the list of ncsa format specifiers in one location only

Other minor cleanups