Handle :normal! fold restore in session file.

A recent Vim 7.3 patch fixed the use of :normal to :normal! zo (so that mappings do not apply). Adapt the plugin's parsing to accommodate this.
Invoke autoload functions instead of defined custom commands.

Some people using the :Session... alternatives may :delcommand the original commands; in that case, the invocations by the script would fail. Prevent this error by invoking the autoload functions directly; this also obsoletes the need for fnameescape().
Skip the "save modified session" confirmation when re-opening the same.

When the current session is re-:OpenSession'ed again, the intention of the user is to revert to the saved state. No need to ask for persistence of the modifications, then. (This is like :edit! on a Vim buffer.)
Merge remote-tracking branch 'inkarkat/avoid-own-command-use'
Merge remote-tracking branch 'inkarkat/handle-changed-normal-persistence'
Merge remote-tracking branch 'inkarkat/avoid-confirm-when-reopening'