Upgrade to new servald, fix ServalD encapsulation

Servald command line "set did" is now "keyring set did" with same output
format as "keyring add"

All servald keyring PIN arguments are now passed as optional
--entry-pin=<pin> arguments

Encapsulate all servald command lines into ServalD class, make
ServalD.command() methods protected so that command-line details remain
local to ServalD.java

Rename Identity.sid to Identity.subscriberId