os/exec: make LookPath always search the current directory under Windows.
os/exec: make LookPath always search the current directory under Windows.

cmd.exe implicitly looks in "." before consulting PATH.
LookPath should match this behavior.

R=alex.brainman, rsc
CC=golang-dev
http://codereview.appspot.com/5434093

Committer: Alex Brainman <54ba58c8441b5eff861de6b4e5ee46baf086b142@gmail.com>
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go