add path.Clean and other utilities.

use path.Clean in web server to sanitize URLs.

http://triv/go/../../../etc/passwd

no longer serves the password file.
it redirects to

http://triv/etc/passwd

which then gets a 404.

R=r
DELTA=288  (286 added, 0 deleted, 2 changed)
OCL=27142
CL=27152
4 files changed
tree: d99b97d1215df17d8a4f968eb03664bc6512dee7
  1. doc/
  2. include/
  3. lib/
  4. pkg/
  5. src/
  6. test/
  7. usr/