Prepare for upcoming kernel changes:

• Support for usbonly (SL4) security level (#75)
• Support for boot sysfs device attribute (#76)

DBus API changes:

• BoltStatus was split (#81), so that:
  • Device.Status does not report authorized-xxx anymore
  • Device.AuthFlags added to indicate auth details, e.g. secure, nopci, boot, nokey (#76)
• BoltSecurity and thus Manager.SecurityLevel can report usbonly (#75)

client/boltctl:

• async versions for many function calls
• more efficient getters, resulting in reduced allocations
• boltctl reports Device.AuthFlags
• boltctl prints more and better version info via boltctl monitor

Other bugfixes and improvements include:

• more robust flags/enum conversion