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.

Change-Id: Ic1c0437d2438c6e19b8fff8278ecda9b96bd020b
Reviewed-on: http://gerrit.openafs.org/8920
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>