SOLARIS: Use vn_renamepath as early as possible

Commit 6c509601 uses the vn_renamepath when we are building on Solaris
11. However, some recent patch level of Solaris 10 (more recent than
stock 10u10) has the same problem fixed by that commit, where
vn_setpath takes an additional argument. So instead, just test for the
existence of vn_renamepath itself, so we also use it on Solaris 10
when we can.

Thanks to Rich Sudlow for reporting this.

Reviewed-on: http://gerrit.openafs.org/8920
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 1b63689b99b49d902dd5a3286b14dcccee88b4a2)

Change-Id: I035c76dba51571fa82fd8932302e7f26b4954333
Reviewed-on: http://gerrit.openafs.org/8984
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>