common: Rename cockpit/ subdirectory to common/

For new contributors it's confusing to have a directory
named 'cockpit'.

And clean up the last vestiges of libcockpit being an installable
library.

Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>
common: Rename libcockpit.a to libcockpit-common.a

Just to make things clearer

Closes #786
Reviewed-by: Marius Vollmer <marius.vollmer@redhat.com>