MEDIUM: tools: make str2sa_range() validate callers' port specifications

MEDIUM: tools: make str2sa_range() validate callers' port specifications

Now str2sa_range() will enforce the caller's port specification passed
using the PA_O_PORT_* flags, and will return an error on failure. For
optional ports, values 0-65535 will be enforced. For mandatory ports,
values 1-65535 are enforced. In case of ranges, it is also verified that
the upper bound is not lower than the lower bound, as this used to result
in empty listeners.

I couldn't find an easy way to test this using VTC since the purpose is
to trigger parse errors, so instead a test file is provided as
tests/ports.cfg with comments about what errors are expected for each
line.