Improvements on global variables , issue #67

init_global_variables() has a new input parameters: runtime
init_global_variables() exits with EXIT_FAILURE only if runtime==0
Added function config_file_is_readable()
Added function init_signal_handlers()
Introduced sighup_handler() but not enabled yet
Added "debug" in [global] section
Fixed a typo in init_global_variables() where, to enable gdbg_lvl, [mysql users] was checked instead of [debug]
Global mutexes in init_global_variables() are initialized only if runtime==0
Removed some code that was commented from long time
Disabled the code that enables gdbg (that was bugged) and is now managed globally
Removed the unused variable print_statistics_interval