[bootsvc] Support process arguments for bootsvc.next

Update bootsvc.next to support optionally specifying arguments to the
process launched by bootsvc, delimited by commas.

Note that the bootsvc tests still only run manually, though we are
investigating running them automated on CQ similar to the core-tests.
Tracked in ZX-4000.

ZX-3662 #done
ZX-4000 #comment

Test: fx --dir out/x64 build && ./zircon/scripts/run-zircon-x64 -o
out/x64.zircon -c "bootsvc.next=bin/bootsvc-tests,testargument"

Change-Id: Iecfdf22be93c8d0893ffc1b58d541a2b3ad7e97c