CLEANUP: cfgparse-global: move the log keywords to a single parser

CLEANUP: cfgparse-global: move the log keywords to a single parser

"log", "log-send-hostname" and "log-tag" are all related to the logging
setup, so they were moved together to a new cfg_parse_global_log_opts()
function and registered in cfg_kws so as to become discoverable via
"-dKcfg". The "log" keyword uses cfg_curr_kwm to detect "no log".

Like for previous commits, the result of the strdup() of the hostname
is now checked, and the duplicate hostname only emits a warning.