Switch man pages from SGML to RST

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>