Introduce a new BoltIdent struct that holds the name and vendor
information for a thunderbolt device together with a reference
to the 'struct udev_device' that provides those information.
This binds the lifetime of the char pointers directly to the
struct where they are contained and thus prevent accidental
bugs where the strings are used after the udev_device is not
valid anymore (got un-referenced).
Since the struct holds a reference, define a clear method for
it and the corresponding auto-cleanup method.
Use the new BoltIdent struct for 'bolt_device_ident'. Re-factor
it to not use a loop anymore.

The read_sysattr_name helper, use in 'bolt_device_ident' now
uses the recently introduced sysattr value getter.