Kernel+LibC: Do not return -ENAMETOOLONG from sys$readlink()

That's not how readlink() is supposed to work: it should copy as many bytes
as fit into the buffer, and return the number of bytes copied. So do that,
but add a twist: make sys$readlink() actually return the whole size, not
the number of bytes copied. We fix up this return value in userspace, to make
LibC's readlink() behave as expected, but this will also allow other code
to allocate a buffer of just the right size.

Also, avoid an extra copy of the link target.

Libraries/LibC/unistd.cpp

@@ -364,7 +364,8 @@ ssize_t readlink(const char* path, char* buffer, size_t size)
 {
     Syscall::SC_readlink_params params { { path, strlen(path) }, { buffer, size } };
     int rc = syscall(SC_readlink, &params);
-    __RETURN_WITH_ERRNO(rc, rc, -1);
+    // Return the number of bytes placed in the buffer, not the full path size.
+    __RETURN_WITH_ERRNO(rc, min((size_t)rc, size), -1);
 }
 
 off_t lseek(int fd, off_t offset, int whence)