Change how client option -s interprets ':'

Change how client option -s interprets ':'

Old version recognizes the first ':', which prevents use of ':' in
host names.  They are used in numerical IPv6 addresses.  New version
recognizes the last ':', which prevents use of ':' in service names.

Old version treats empty host or port specially (use default).
Documentation suggests ':' is required, but the code doesn't do that.
Instead, the argument is interpreted as host, even when it's empty.

New version makes the HOST: part optional.  You can't specify host and
default the port.  Tough.  Keeps documentation and code as simple as
possible.

Compare:

                old version             new version
    argument    host    port            host    port
    ""          ""      default         default ""
    "A"         "A"     default         default "A"
    ":"         default default         ""      ""
    "A:"        "A"     default         "A"     ""
    ":B"        default "B"             ""      "B"
    "A:B"       "A"     "B"             "A"     "B"