avoid multiple ikiwiki cgi processes piling up, eating all memory, and thrashing

avoid multiple ikiwiki cgi processes piling up, eating all memory, and thrashing

Fixed by making the cgi wrapper wait on a cgilock.
If you had to set apache's MaxClients low to avoid ikiwiki thrashing
your server, you can now turn it up to a high value.

The downside to this is that a cgi call that doesn't need to call lockwiki
will be serialised by this so only one can run at a time. (For example,
do=search.) There are few such calls, and all of them call loadindex,
so each still eats gobs of memory, so serialising them still seems ok.