canonicalize_path: initdb.c -> port/path.c

For application to HEAD.

The win32 port backend will require the functionality provided by
canonicalize_path. Patch moves it from initdb.c to port/path.c.

---
Certain disclaimers and policies apply to all email sent from Memetrics.
For the full text of these disclaimers and policies see
<a
href="http://www.memetrics.com/emailpolicy.html">http://www.memetrics.com/em
ailpolicy.html</a>