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.

Reviewed-on: http://gerrit.openafs.org/3305
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 5c77a6ca2460c3e79125a614037824db35480905)

Change-Id: I23d7af2dd07e02255783de0523fce58dbe46d31e
Reviewed-on: http://gerrit.openafs.org/6267
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>