clean up APIs for setting a request or response as being cacheable

redundant Set in the name function name for the Set*CacheableSet and not having the ability to turn off caching from the API:
TSHttpTxnSetReqCacheableSet(TSHttpTxn txnp)
TSHttpTxnSetRespCacheableSet(TSHttpTxn txnp)

should be:
TSHttpTxnReqCacheableSet(TSHttpTxn txnp, int flag)
TSHttpTxnRespCacheableSet(TSHttpTxn txnp, int flag)

to be like other API function prototypes:
TSSkipRemappingSet(TSHttpTxn txnp, int flag);