libc: update O_APPEND fds on joined child

The fds in O_APPEND state may have changed by the child. The local seek
state needs to be valid/current for the next to be forked child, which seek
state is set by File_descriptor_allocator::generate_info().

Issue #3991

repos/libports/include/libc-plugin/fd_alloc.h

@@ -113,6 +113,11 @@ namespace Libc {
 			 */
 			File_descriptor *any_cloexec_libc_fd();
 
+			/**
+			 * Update seek state of file descriptor with append flag set.
+			 */
+			void update_append_libc_fds();
+
 			/**
 			 * Return file-descriptor ID of any open file, or -1 if no file is
 			 * open