Issue 1013 - update atime on exec and mmap, add helper vop_markatime

Issue 1013 - update atime on exec and mmap, add helper vop_markatime

Add a new helper VOP which passively updates a vnode's atime.  Implement
the new VOP in HAMMER and UFS and create a default op which does nothing.
Call the new function from mmap() and exec().

Submitted-by: Dion Blazakis
Reported-by: Simon Schubert