CLEANUP: cfgparse-global: move the process-wide limits to a single parser

CLEANUP: cfgparse-global: move the process-wide limits to a single parser

"maxconn", "maxconnrate", "maxpipes", "maxsessrate", "maxsslrate",
"fd-hard-limit" and "ulimit-n" all take a single integer argument, may
only be specified once and simply assign a field in the global struct,
so they were moved together to a new cfg_parse_global_limits() function
and registered in cfg_kws so as to become discoverable via "-dKcfg".

The arguments are now checked before the value is assigned rather than
after. The two non-fatal alerts ("already specified" and "maxconn too high")
are emitted directly via ha_warning() / ha_alert(). (Same as for previous
patch, "already specified" turned to a warning) since more appropriate.