This commit sets prgname correctly so that the "ostree subcommand
--help" output prints the subcommand rather than just "ostree".
This was removed in commit f0519e541f because it tripped the thread
sanitizer, but it's being added back conditionally so most users who
don't compile with -fsanitize=adress see proper help output.
Closes: #1054
Approved by: cgwalters