terminal-util: introduce openpt_allocate()

Allocating a pty is done in a couple of places so let's introduce a new helper
which does the job.

Also the new function, as well as openpt_in_namespace(), returns both pty
master and slave so the callers don't need to know about the pty slave
allocation details.

For the same reasons machine_openpt() prototype has also been changed to return
both pty master and slave so callers don't need to allocate a pty slave which
might be in a different namespace.

Finally openpt_in_namespace() has been renamed into
openpt_allocate_in_namespace().

src/machine/machine.c

@@ -530,29 +530,20 @@ int machine_kill(Machine *m, KillWho who, int signo) {
         return manager_kill_unit(m->manager, m->unit, signo, NULL);
 }
 
-int machine_openpt(Machine *m, int flags) {
+int machine_openpt(Machine *m, int flags, char **ret_slave) {
         assert(m);
 
         switch (m->class) {
 
-        case MACHINE_HOST: {
-                int fd;
+        case MACHINE_HOST:
 
-                fd = posix_openpt(flags);
-                if (fd < 0)
-                        return -errno;
-
-                if (unlockpt(fd) < 0)
-                        return -errno;
-
-                return fd;
-        }
+                return openpt_allocate(flags, ret_slave);
 
         case MACHINE_CONTAINER:
                 if (m->leader <= 0)
                         return -EINVAL;
 
-                return openpt_in_namespace(m->leader, flags);
+                return openpt_allocate_in_namespace(m->leader, flags, ret_slave);
 
         default:
                 return -EOPNOTSUPP;