page: implement bookmarks (thanks trav_ for starting it)

add new functions pageaddr() that returns a string describing
the page to be loaded. it is in the form of:

/path/to/file!pagename!subpage!....

one can jump to such a page by calling trywalk(name, addr)
where name and addr get concatinated with ! to form a page
address and then the currently loaded pages are walked up
to the nearest page which is then returned. (or nil when
not found). the remaining address will be set in the global
pagewalk variable.

once pages get loaded (asynchronously), pagewalk1() gets
called again on addpage() and continues the walking up to the
last page.

new program flag -j <addr> was added to jump to a page on
startup.

page address (without filename) can also be supplied in
plumb message with the "addr" attribute.

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