rump_fs/fuse_fs/lx_fs/ram_fs: symlink fixup

Allow symlinks to be passed to the read and write file system utilities.

Disallow writes to symlinks with offsets in file system servers, this is
to ensure that writing the target of a symlink is an atomic operation.

Fixes #1604

repos/os/src/server/lx_fs/symlink.h

@@ -39,6 +39,9 @@ class File_system::Symlink : public Node
 
 		size_t write(char const *src, size_t len, seek_off_t seek_offset)
 		{
+			/* Ideal symlink operations are atomic. */
+			if (seek_offset) return 0;
+
 			size_t count = min(len, sizeof(_link_to) + 1);
 			Genode::strncpy(_link_to, src, count);
 			return count;