MINOR: tools: report explicit address type selection

str2sa_range() reports the resulting socket and transport types through
net_addr_type. It does not tell callers whether these types came from
defaults or from an explicit prefix. Callers that need to distinguish
the two must duplicate the parser prefix list, which can fall out of
sync.

Add explicit_type and set it for prefixes that select a socket or
transport type. Family-only prefixes leave it unset. This has no effect
on existing callers.