I happened to notice that if you say "help disable", you get a list
of sub-commands that includes "disable breakpoints", but if you say
"help enable", the list of sub-commands, does NOT include "enable
breakpoints".  It also doesn't tab-complete on "enable br<tab>".

On investigation, I discovered that the "enable breakpoints" command
is instantiated using "add_abbrev_prefix_cmd".  What the heck -- it's
not an abbreviation.  This dates back to before the current cvs repo.

Changing that call to "add_prefix_cmd" fixes the issues, and causes
no regressions when tested on amd64-linux.

OK to commit?