BUG/MINOR: config: don't emit a warning when global stats is incompletely configured

BUG/MINOR: config: don't emit a warning when global stats is incompletely configured

Martin Brauer reported an unexpected warning when some parts of the
global stats are defined but not the listening address, like below :

  global
    #stats socket run/admin.sock mode 660 level admin
    stats timeout 30s

Then haproxy complains :
  [WARNING] 334/150131 (23086) : config : frontend 'GLOBAL' has no
'bind' directive. Please declare it as a backend if this was intended.

This is because of the check for a bind-less frontend (the global section
creates a frontend for the stats). There's no clean fix for this one, so
here we're simply checking that the frontend is not the global stats one
before emitting the warning.

This patch should be backported to all stable versions.