sfdisk: replace my_warn() with warnx()

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 can break scripts, as someone might have
wrote automation which expects output in the format it has always been.
In programmer terms, the change has an ABI change, which some may find
annoying.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>