Windows: refactor fs, symlink and fs_utils

Windows: refactor fs, symlink and fs_utils

over the years a large number of duplicated functionality
has been added to symlink.c and fs.c.  Refactor both so all
common functionality is implemented within fs_utils.c.

Ensure that all functionality migrated to fs_utils.c and
symlink.c uses the SafeString library functions.

Update the build rules for afs_shl_ext.dll, afscreds.exe
and afsconfig.exe to support the changes to fs_utils.c.

Remove non-windows components from symlink.c.

Change-Id: Ia7709235600f48c09282246e23b1f13d26f54e28
Reviewed-on: http://gerrit.openafs.org/4777
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>