fix --help output formatting

fix --help output formatting

For the sake of having the string indented, we use
backslash-line-continuations, but they swallow up all the spaces at
the beginning of the next line, so we use `\n  \` explicitly with
spaces between the newline and the line-continuation to introduce the
indentation in the output.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>