Set execute bit on files if and only if they begin with (#!). (#7347)

* Set execute bit on files if and only if they begin with (#!).

Git only tracks the 'x' (executable) bit on each file. Prior to this
CL, our files were a random mix of executable and non-executable.
This change imposes some order by making files executable if and only
if they have shebang (#!) lines at the beginning.

We don't have any executable binaries checked into the repo, so
we shouldn't need to worry about that case.

* Added fix_permissions.sh script to set +x iff a file begins with (#!).

Author: haberman