[musl][dl] provide dl_clone_loader_service()

If there is an active loader service and it supports obtaining
a new connection, this function will return a new connection.

Change-Id: I58fe4d2450dc656340ed352424ef095ad19227fe

third_party/ulib/musl/include/magenta/dlfcn.h

@@ -15,6 +15,12 @@ void* dlopen_vmo(mx_handle_t, int);
 // in the return value.
 mx_handle_t dl_set_loader_service(mx_handle_t new_svc);
 
+// Ask the active "loader service" (if there is one), to return
+// a new connection.  Not all loader services need support this.
+// On success, a channel handle to the new connection is returned
+// via out.
+mx_status_t dl_clone_loader_service(mx_handle_t* out);
+
 #ifdef __cplusplus
 }
 #endif

third_party/ulib/musl/ldso/dynlink.c

@@ -2252,6 +2252,23 @@ __NO_SAFESTACK static mx_status_t get_library_vmo(const char* name,
                           MX_HANDLE_INVALID, result);
 }
 
+__NO_SAFESTACK mx_status_t dl_clone_loader_sevice(mx_handle_t* out) {
+    if (loader_svc == MX_HANDLE_INVALID) {
+        return MX_ERR_UNAVAILABLE;
+    }
+    mx_handle_t h0, h1;
+    mx_status_t status;
+    if ((status = _mx_channel_create(0, &h0, &h1)) != MX_OK) {
+        return status;
+    }
+    if ((status = loader_svc_rpc(LOADER_SVC_OP_CLONE, NULL, 0, h1, NULL)) != MX_OK) {
+        _mx_handle_close(h0);
+    } else {
+        *out = h0;
+    }
+    return status;
+}
+
 __NO_SAFESTACK static void log_write(const void* buf, size_t len) {
     // The loader service prints "header: %s\n" when we send %s,
     // so strip a trailing newline.