gdb/mi: add --no-connection to MI -add-inferior command

gdb/mi: add --no-connection to MI -add-inferior command

Following on from the previous commit, where the -add-inferior command
now uses the same connection as the current inferior, this commit adds
a --no-connection option to -add-inferior.

This new option matches the existing option of the same name for the
CLI version of add-inferior; the new inferior is created with no
connection.

I've added a new 'connection' field to the MI output of -add-inferior,
which includes the connection number and short name.  I haven't
included the longer description field, this is the MI after all.  My
expectation would be that if the frontend wanted to display all the
connection details then this would be looked up from 'info
connection' (or the MI equivalent if/when such a command is added).

The existing -add-inferior tests are updated, as are the docs.