Standardize on --long-option instead of -long-option

Standardize on --long-option instead of -long-option

[[forum/refresh_and_setup]] indicates some confusion between --setup
and -setup. Both work, but it's clearer if we stick to one in
documentation and code.

A 2012 commit to [[plugins/theme]] claims that "-setup" is required
and "--setup" won't work, but I cannot find any evidence in ikiwiki's
source code that this has ever been the case.