Improve timers and fix recursive call in vfs_open

[Galfurian]

• Improve readability of timer code.
• Fix bug with vfs_open, to avoid infinite call loop.

[fischerling]

I think the vfs_open fix is not correct.

Absolute paths containing symlinks are no longer resolved when passing them to vfs_open.
open("/link/bar") with /link pointing to /foo will no longer resolved to /foo/bar.

This can be reproduced by running:

$ cat /user/home/README

[fischerling]

I have a fix ready and will create created a new Pull Request.

See #76 .