MINOR: cfgparse: Parse scope lines and save the last one parsed

MINOR: cfgparse: Parse scope lines and save the last one parsed

A scope is a section name between square bracket, alone on its line, ie:

  [scope-name]
  ...

The spaces at the beginning and at the end of the line are skipped. Comments at
the end of the line are also skipped.

When a scope is parsed, its name is saved in the global variable
cfg_scope. Initially, cfg_scope is NULL and it remains NULL until a valid scope
line is parsed.

This feature remains unused in the HAProxy configuration file and
undocumented. However, it will be used during SPOE configuration parsing.