[nfs] - fix excessive calls to mount_getexports - only call it when the
hostname changes (as it was intended in the first place) by not clearing
the list whenever the path changes
[win32/nfs] - bump libnfs to 1.6.2 again for windows as there is a
workaround for the issues now
Merge pull request #4545 from Memphiz/nfsexportlist

[win32] - nfs fix