Added two new params: html.head.legalnotice.link.types and

Added two new params: html.head.legalnotice.link.types and
html.head.legalnotice.link.multiple.

If the value of the generate.legalnotice.link is non-zero, then
the stylesheet generates (in the head section of the HTML source)
either a single HTML link element or, if the value of the
html.head.legalnotice.link.multiple is non-zero, one link element
for each link type specified. Each link has the following
attributes:

  - a rel attribute whose value is derived from the value of
    html.head.legalnotice.link.types

  - an href attribute whose value is set to the URL of the file
    containing the legalnotice

  - a title attribute whose value is set to the title of the
    corresponding legalnotice (or a title programatically
    determined by the stylesheet)

For example:

  <link rel="copyright" href="ln-id2524073.html" title="Legal Notice">

Closes #1476450. Thanks to Sam Steingold.