do not allow overriding the header/footer settings for cover pages

This was discovered in #1676, where using a command-line like:

  wkhtmltopdf cover cover.html --header-html header.html page.html output.pdf

would override the settings for the cover page and not apply the
--header-html for the URL page.html. This may be unexpected behavior
for most users, but the manual clearly states that the options FOLLOW
the URL/file name, as in:

  cover <input url/file name> [PAGE OPTION]...
  (page)? <input url/file name> [PAGE OPTION]...

So, in the above example it actually means the following

  URL1: cover cover.html --header-html header.html
  URL2: page.html
  OUT:  output.pdf

To fix this, we parse the other page options for a cover page and
then set the header/footer settings to blank values to ensure that
they are not shown on a cover page.
set page margins correctly via header/footer heights for multiple URLs

In 80cbcc5db95eb48127a2d1418653e8643ad74f46, the ability for
auto-calculation of the header/footer height was implemented.
However, in case of multiple page objects (i.e. URLs) being present
for conversion, the calculated height for only the FIRST page object
was used -- the heights for the second and further page objects were
being ignored. In case a cover page was specified first (which has
no header/footer by design) without an explicit margin, then the
top/bottom margins would be set to zero -- which would hide the
header/footer content.

The solution is to use the maximum value across all the page objects,
so that there is enough space for the header content across all page
objects. Fixes #1676