Rename to BUFFER->pathbuf to make it clear the field is a BUFFER, and
to make sure to catch and review all usages.

There are still uses of pathbuf that are truncating and need to be
fixed, for example in browser.c and buffy.c.

Fix up sidebar usage in one place, that was pointing inside the
BUFFY->path with a char *.  At the same time, change their "short
folder" computation to also use a BUFFER.