BUG/MINOR: stats: fixing stat shows disabled frontend status as 'OPEN'

BUG/MINOR: stats: fixing stat shows disabled frontend status as 'OPEN'

This patch adresses the issue #1626.

Adding support for PR_FL_PAUSED flag in the function stats_fill_fe_stats().
The command 'show stat' now properly reports a disabled frontend
using "PAUSED" state label.

This patch depends on the following commits:
  - 7d00077fd5 "BUG/MEDIUM: proxy: ensure pause_proxy()
  and resume_proxy() own PROXY_LOCK".
  - 001328873c "MINOR: listener: small API change"
  - d46f437de6 "MINOR: proxy/listener: support for additional PAUSED state"

It should be backported to 2.6, 2.5 and 2.4