Feat: Default PID

Make the pid parameter default to the PID of the current process.

Change the exit code for invalid arguments to a value in the user
defined range (see https://tldp.org/LDP/abs/html/exitcodes.html). This
may be considered a breaking change, but since the only applications
that currently use this script don't rely on this code, I don't think
we need to bump the major version for this.

rpid

@@ -3,7 +3,8 @@
 USAGE='
 USAGE: rpid <pid|-h|-[-]help> [<root_name>=login]
 
-        pid     The PID of the process for which you want to find the root PID.
+        pid     The PID of the process for which you want to find the root
+                PID. Defaults to $$.
 
         root_name
                 The name (not command line) of the root process, for which
@@ -20,16 +21,16 @@ EXAMPLES:
         Get the PID of the login process that is the ancestor of the current
         process.
 
-                rpid $$
+                rpid
 '
 
 # Validate the arguments
-if [[ "$#" -lt 1 || "$#" -gt 2 ]]; then
+if [[ "$#" -gt 2 ]]; then
   printf '%s' "${USAGE}" 1>&2
-  exit 128
+  exit 64
 fi
 
-pid="$1"
+pid="${1:-$$}"
 
 # Check if the user needs help
 if [[ "${pid}" =~ ^(-h|-(-)?help)$ ]]; then