tree-wide: add short comments for namespace_open() and namespace_enter()

Also use -EBADF when unspecified.

src/basic/namespace-util.c

@@ -243,7 +243,12 @@ int userns_acquire(const char *uid_map, const char *gid_map) {
         if (r < 0)
                 return log_error_errno(r, "Failed to write GID map: %m");
 
-        r = namespace_open(pid, NULL, NULL, NULL, &userns_fd, NULL);
+        r = namespace_open(pid,
+                           /* ret_pidns_fd = */ NULL,
+                           /* ret_mntns_fd = */ NULL,
+                           /* ret_netns_fd = */ NULL,
+                           &userns_fd,
+                           /* ret_root_fd = */ NULL);
         if (r < 0)
                 return log_error_errno(r, "Failed to open userns fd: %m");
 

src/basic/terminal-util.c

@@ -1211,7 +1211,7 @@ int openpt_allocate_in_namespace(pid_t pid, int flags, char **ret_slave) {
 
         assert(pid > 0);
 
-        r = namespace_open(pid, &pidnsfd, &mntnsfd, NULL, &usernsfd, &rootfd);
+        r = namespace_open(pid, &pidnsfd, &mntnsfd, /* ret_netns_fd = */ NULL, &usernsfd, &rootfd);
         if (r < 0)
                 return r;
 
@@ -1262,7 +1262,7 @@ int open_terminal_in_namespace(pid_t pid, const char *name, int mode) {
         pid_t child;
         int r;
 
-        r = namespace_open(pid, &pidnsfd, &mntnsfd, NULL, &usernsfd, &rootfd);
+        r = namespace_open(pid, &pidnsfd, &mntnsfd, /* ret_netns_fd = */ NULL, &usernsfd, &rootfd);
         if (r < 0)
                 return r;