[[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.
If someone has explicitly disabled the postform, it seems reasonable
from a least-astonishment point of view for that to take precedence
over rootpage, even though that makes rootpage useless.
Also add a regression test; so far, this is all it tests.
Simon McVittie