common: check_create_dir(): Support symlinks as well.

Previously, the following situation would cause an endless look (as reported
by Michael Prokop in Debian bug #619123): the (CSV or RRD) datadir is a
symlink pointing to a non-existent target.

With this patch applied, check_create_dir() fails with "<file> exists but is
not a directory".

Signed-off-by: Sebastian Harl <sh@tokkee.org>