cf-serverd: fix file-access path calculation, after symlink expansion

If we define an access rule to some symlink, like:
 "/var/cfengine/masterfiles" -> "/var/lib/cfengine/masterfiles"
then, pre-processing of the path will expand it to a longer string,
and thus "path_len" will be invalid.
Merge pull request #1558 from xrg/fix-server-access-promise

cf-serverd: fix file-access path calculation, after symlink expansion