STABLE14-windows-robust-freelance-20041126

STABLE14-windows-robust-freelance-20041126

Implement new functions: cm_freelanceMountPointExists and
cm_freelanceSymlinkExists.  Use them along with other validity checks
in cm_freelanceAddMount and cm_freelanceAddSymlink to ensure that name
collisions do not occur and that empty strings are not valid file names.

A symlink may not have a name which would resolve to a valid cell name.
Doing so would prevent access to the cell.

(cherry picked from commit 3c487ecb59c0bace3a417245f639f7e3388ae5dc)