Avoid duplication of sender subcommands.

Avoid duplication of sender subcommands.

The list of sender subcommands (add, delete, allow, deny, on, off)
is defined as an enumeration in send.h. This list is duplicated in
check_sender_args() of command.c which contains the six subcommands
as C-strings to be matched against the first word of the sender
command line.

For the code to work properly it is essential that the two subcommand
lists are identical, which is a bad design that is quite error
prone. Fortunately it is easy to avoid the duplication with a little
preprocessor fu.

Since the subcommands are spelled in lower case and there is no
toupper function in CPP, we need to change the subcommand part of the
enumeration constants to lower case. The bulk of the patch consists
in trivial changes of all the users of these constants.