src/server/conf.c: Add missing function prototype

Forward declare the host_to_command() function.
src/server/auth.c: Remove code duplication

Use a single loop to match the configured "hosts", "services", and
"commands" patterns against submitted commands.  This might also speed
up the code a tiny bit, as cfg_getopt() is now called only once (by us)
instead of twice (by libConfuse).