Switch man pages from SGML to RST

This patch changes the Makefile rules to build the man pages from the
RST sources instead of the SGML ones.

Note: pandoc quotes by default the ‘@’ character in manpage output, so
we explicitely undo this before writing the .in file.

Note 2: The docs unittest has switched from checking cmdsynopsis
elements to search for either '**command**' or '| **command**' at the
beggining of lines. This is not as safe as the sgml checker, but it
should be fine.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>