Fix bug when redoing multiple (agenda) blocks.

Fix bug when redoing multiple (agenda) blocks.

* org-agenda.el (org-agenda-overriding-cmd)
(org-agenda-multi-current-cmd)
(org-agenda-multi-overriding-arguments): New variables.
(org-agenda-run-series): `org-agenda-overriding-arguments'
defaults to the last agenda block arguments, so don't use it
globally.
(org-agenda-mark-header-line): Add properties needed so that
`org-agenda-overriding-arguments', `org-agenda-current-span'
and `org-agenda-last-arguments' can be set to their correct
contextual value.
(org-agenda-multi-back-to-pos): New variable.
(org-agenda-later): Retrieve `org-agenda-current-span' and
`org-agenda-overriding-arguments' from text properties.  Also
handle numeric span.
(org-agenda-later, org-agenda-change-time-span): Set
`org-agenda-overriding-cmd' so that we to take overriding
arguments into account for this command only.

The behavior for agenda blocks where there is only one (agenda)
command is not changed.  Changing the time span and redoing with
`g' will keep the new time span.  The behavior changed for blocks
where there are multiples (agenda) blocks.  In this case, redoing
will restore the initial view (before any time span change) and
changing the time span is done independantly for each block.

Thanks to Charles Philip Chan for reporting this nasty bug.