BUG/MINOR: unix: properly check for octal digits in the "mode" argument

BUG/MINOR: unix: properly check for octal digits in the "mode" argument

A config containing "stats socket /path/to/socket mode admin" used to
silently start and be unusable (mode 0, level user) because the "mode"
parser doesn't take care of non-digits. Now it properly reports :

   [ALERT] 276/144303 (7019) : parsing [ext-check.cfg:4] : 'stats socket' : ''mode' : missing or invalid mode 'admin' (octal integer expected)'

This can probably be backported to 1.7, 1.6 and 1.5, though reporting
parsing errors in very old versions probably isn't a good idea if the
feature was left unused for years.