RAPI: Add flag to require authentication

Most RAPI resources do not require authentication for the “GET” method.
In some setups it can be desirable to always require authentication.
This patch adds a command line parameter to always require it.

Some unrelated minor typos in the “ganeti-rapi” man page are also fixed.

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