Generate the VSL record tag documentation from the header tables.

This patch generates an rst file containing the record tag
documentation from the corresponding header tables.

It also adds a man-page from the vsl.rst document and includes the tag
descriptions.

The old list of tags in varnishlog.rst is removed.

Fixes: #1338
Remove xid field from ReqEnd description

The xid field is not present in ReqEnd any more, as it is not needed
now that the vxid is used as the record ID. Remove it from the
description.
Reformat the log record long descriptions.

Format the long log record descriptions for use with pre-formatted
text style.
Move the TTL and Gzip log record description to vsl_tags.h
Honor RST definition list format in long description of log records.