proc_arst - detect asynchronous resets

yosys> help proc_arst
proc_arst [-global_arst [!]<netname>] [selection]
This pass identifies asynchronous resets in the processes and converts them
to a different internal representation that is suitable for generating
flip-flop cells with asynchronous resets.
-global_arst [!]<netname>
In modules that have a net with the given name, use this net as async
reset for registers that have been assign initial values in their
declaration ('reg foobar = constant_value;'). Use the '!' modifier for
active low reset signals. Note: the frontend stores the default value
in the 'init' attribute on the net.