sfdisk: replace my_warn() with warnx()

This change adds program name in front of error messages.  Often that
helps user to know which command sent message, when the command is
executed as part of script.

Unfortunately the prefixing may break scripts, as there is no knowledge
whether some scripts are expecting not command name prefixed error
messages from this utility.  Same in programmer terms; the change
contains ABI change.

Karel: could you sign-off abi change, if not please drop this patch.
Signed-off-by: Sami Kerola <kerolasa@iki.fi>