comerr: long and short form of the -prefix option.

The man page documented the prefix option as -prefix; compile_et
supported only the short form -p.

Document and support both the long and short forms for the prefix
option; -p and -prefix.

Change-Id: Ide5551b06ae888748600677ed09ba674506a584f
Reviewed-on: http://gerrit.openafs.org/10721
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>