include/ipc: Clean up ipc_msg API

include/ipc: Clean up ipc_msg API

The ipc module currently doesn't provide functions to get all fields
contained within a message. Further, signature-related functions don't
return an error if the message is invalid or the signature could not be
retrieved for another reason.
This commit adds the missing functions to the public API of the ipc
module and fixes the behavior of signature related functions so the
caller can correctly determine if an error has occurred.

The following functions have been added:
 - ipc_msg_get_version()
 - ipc_msg_get_signature()
 - ipc_msg_get_signer_name()
 - ipc_msg_get_signer_email()
 - ipc_msg_get_signer_key()