utime fixes

http://pubs.opengroup.org/onlinepubs/009695399/functions/utime.html

From browsing the linux kernel source, it appears when setting inode attributes that permissions are only checked if the uid of the inode doesn't match the uid associated with the calling process. In emscripten's case, this is always true currently.

In similar vein to the other pull requests, I replaced the test with one with no JS API dependencies.