Properly complain if people try to reuse symbols.

Fixes #1510
Remove a now pointless assert.

Pointed out by:	Dridi Boukelmoune
Verify that s_resp_bodybytes is indeed incremented the correct number of bytes on partial content responses.
Add a message about persistent storage being deprecated now.
Rename -spersistent to make it clear that it is deprecated.
Minor cleanup of how we write the warning at the top of generated files.
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
Beautify the output of the vmodtool to make it (also) human readable.