rxgen: add *TranslateOpcode functions to XDR code

rxgen: add *TranslateOpcode functions to XDR code

Administrative commands and server binaries can benefit from the
ability to translate RPC OpCode numbers to proper function names.

For this purpose, rxgen now emits the function

char *PKGTranslateOpCode(int)

where PKG is replaced by the respective package prefix.

The function is generated in the exact same way that ExecuteRequest is.
This way, the right optimization (array storage vs. generated case
statement) according to the respective sparseness of OpCodes in the
different RX packages is maintained.

Change-Id: Ib083597c0841f50518afd2b159b98c0447e13da0
Reviewed-on: http://gerrit.openafs.org/3305
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>