scripts/get_abi.pl: add a validate command

Sometimes, we just want the parser to retrieve all symbols from
ABI, in order to check for parsing errors. So, add a new
"validate" command.

While here, update the man/help pages.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>