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.

Reviewed-on: http://gerrit.openafs.org/10721
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 37937a3e924eee7a09c7719ca6355703e65092e1)

Change-Id: I65b8d54bde89be85bd535d3a85fd2b22408bba39
Reviewed-on: http://gerrit.openafs.org/11225
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>