Simplify Perforce command definition.
Move common input logic into defp4cmd*, enabling more commands to be converted (p4-revert, p4-sync, p4-have, p4-changes, p4-resolve, p4-opened, p4-jobs, p4-fixes, p4-clients, p4-branches, p4-labels, p4-login).
Move completion-type argument to end of p4-read-args so that we can use the default value more often.