MINOR: tools: make str2sa_range() directly return the protocol

MINOR: tools: make str2sa_range() directly return the protocol

We'll need this so that it can return pointers to stacked protocol in
the future (for QUIC). In addition this removes a lot of tests for
protocol validity in the callers.

Some of them were checked further apart, or after a call to
str2listener() and they were simplified as well.

There's still a trick, we can fail to return a protocol in case the caller
accepts an fqdn for use later. This is what servers do and in this case it
is valid to return no protocol. A typical example is:

   server foo localhost:1111