CLEANUP: cfgparse-global: move "uid", "gid", "user" and "group" to its own parser

CLEANUP: cfgparse-global: move "uid", "gid", "user" and "group" to its own parser

These 4 keywords all end up setting global.{uid,gid}, either from a
number or by resolving a system user or group name, so they were moved
together to a new cfg_parse_global_uid_gid() function and registered in
cfg_kws so as to become discoverable via "-dKcfg". The parser continues
to use ha_alert() / ha_warning() since keyword parsers can normally only
return a single message (e.g. "statically built libc" could be emitted
before the rest).