process-util: move fork_agent() to process-util.[ch]

It's a relatively small wrapper around safe_fork() now, hence let's move
it over, and make its signature even more alike. Also, set a different
process name for the polkit and askpw agents.

src/shared/spawn-ask-password-agent.c

@@ -40,8 +40,9 @@ int ask_password_agent_open(void) {
         if (!isatty(STDIN_FILENO))
                 return 0;
 
-        r = fork_agent(&agent_pid,
+        r = fork_agent("(sd-askpwagent)",
                        NULL, 0,
+                       &agent_pid,
                        SYSTEMD_TTY_ASK_PASSWORD_AGENT_BINARY_PATH,
                        SYSTEMD_TTY_ASK_PASSWORD_AGENT_BINARY_PATH, "--watch", NULL);
         if (r < 0)