CLEANUP: cfgparse-global: move "unix-bind" to its own parser

CLEANUP: cfgparse-global: move "unix-bind" to its own parser

The "unix-bind" keyword even had its own args loop in cfg_parse_global().
It's now registered in cfg_kws and parsed by cfg_parse_global_unix_bind(),
making it discoverable via "-dKcfg". The code was slightly restructured,
particularly the long chain of "if (...) { ...; continue; }" was simplified
a bit.

Like in a few previous patches, the duplicate test for "prefix" now just
emits a warning, and the result of strdup() is tested.