Add FR_TIMEVAL_BOUND_CHECK

Add FR_TIMEVAL_BOUND_CHECK implementing bounding for struct
timeval values, similarly to FR_INTEGER_BOUND_CHECK. Use it in
home_server_add and client_parse to increase readability.
Fix all remaining config items to use FR_CONF_OFFSET

Add support to the conffile parser for parsing IPv4 addresses in octal and hex format

Add support to conffile parser for parsing prefix type config items

Standardise on using fr_ipaddr_t structures to represent IP addresses in the conffile
code, and other places.
Merge pull request #670 from spbnick/fr_timeval_bound_check

Add FR_TIMEVAL_BOUND_CHECK