Provide an interface that allows GEOM modules to return multiple messages.

The gctl_error() function provides GEOM modules with the ability
to report only a single message. When running with the verbose
flag, commands that handle multiple devices may want to report a
message for each of the devices on which it operates. This commit
adds the gctl_msg() function that can be called multiple times
to post messages. When finished issuing messages, the application
must either call gctl_post_messages() or call gctl_error() to cause
the messages to be reported to the calling process.

Tested by: Peter Holm