nbtree: Rename nbtinsert.c variables for consistency.

Stop naming special area/opaque pointer variables 'lpageop' in contexts
where it doesn't make sense.  This is a holdover from a time when logic
that performs tasks that are now spread across _bt_insertonpg(),
_bt_findinsertloc(), and _bt_split() was more centralized.  'lpageop'
denotes "left page", which doesn't make sense outside of contexts in
which there isn't also a right page.

Also acquire page flag variables up front within _bt_insertonpg().  This
makes it closer to _bt_split() following refactoring commit bc3087b6.
This allows the page split and retail insert paths to both make use of
the same variables.