Add sanity check of tar "uid, "gid" and "mtime" fields

Add sanity check of tar "uid, "gid" and "mtime" fields

Sometimes a bz2 file is identified as tar.bz2, i.e. a chunk of
raw data may look like a tar header (metadata block).
To reduce chance of such false positive, the format of uid, gid
and mtime field is checked. This fields are expected to contain
octal numbers. In fact, also space and '\0' are allowed - perhaps
I am too liberal.