Link man pages in documentation

This patch depends on “Option to include man pages in documentation”. In
the documentation build including man pages, all “:manpage:`…`”
references are converted to links. For man pages not provided by Ganeti,
Sphinx' standard formatting is used.

A small dance is necessary to hook into Sphinx' processing of man page
roles and to generate automatically resolved links. The code converts
“:manpage:`…`” for known man pages to the data structure equivalent of
“:doc:`$name($section) <man-$name>`”. Additionally it checks the section
numbers and formatting of references (in all builds).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>