[core] Fixed SRTO_KM* options setting.

[maxsharabayko]

This PR fixes/changes:

• Rejecting negative values (as not allowed).
• Handling zero value properly (as the default). Previously setting SRTO_KMREFRESHRATE=0 was changing SRTO_KMPREANNOUNCE=0x7fffffff.
• SRTO_KMREFRESHRATE and SRTO_KMPREANNOUNCE and not readable (as was already stated in the docs).
• Updated docs: described the default 0 value meaning.

The default values of both SRTO_KMREFRESHRATE and SRTO_KMPREANNOUNCE is zero. It is treated as:

• SRTO_KMREFRESHRATE = 0x1000000
• SRTO_KMPREANNOUNCE= 0x1000

TODO

• Consider restricting setting and/or getting zero value. Use the actual default instead? Decision: stay compliant to the rest options.
• Fix warning: comparison of integers of different signs: 'const unsigned int' and 'int' [-Wsign-compare]
  if (km_preanno > (kmref - 1) / 2)

[stevomatthews]

No changes.