Patch: minor optimization to exec

[Andrew Doran <ad%netbsd.org@localhost>]

http://www.netbsd.org/~ad/cleanout/exec.diff

- We already cache a good chunk at the start of the file (usually around
  1k), so look in the cached copy before issuing an expensive vn_rdwr().

- Don't unlock vnodes just to relock them again with the next statement. 
  Add exec_read_locked() to assist with this.

Comments?

Andrew