CLEANUP: cfgparse-listen: move the log format keywords to their own parser

CLEANUP: cfgparse-listen: move the log format keywords to their own parser

"log-format", "log-format-sd", "error-log-format" and "unique-id-format"
all assign a log format expression to the proxy and were handled almost
the same way, so they were moved together to proxy_parse_logformat() and
listed in cfg_kws, making them discoverable via "-dKcfg".

All common parts (arg check, allocation, warning about backends being
ignored) are now shared and performed first. The result of strdup() of
the file name is now checked. The code is cleaner and smaller:

   90 insertions(+), 124 deletions(-)