More for d7935941 ITS#7229

Note read-write vs read-only mmap tradeoffs.

Fix abort of child txn

due to a0993354

Otherwise a process that opens and closes an env more than once
will leak mutex handles. (WIN32/BSD)

This is quick and dirty, should be integrated into mdb_stat.

Ignore current cursor pos if it's out of bounds.

Minimizes the number of memcpy's if additional dups are stored.

Append mode now does no key comparisons, input must be in sorted order.
page_split was not updating cursor parents correctly.

Just lookup the DB roots as needed. When many DBs are in use,
most of the copies won't be referenced in a given txn, and
there's a bad race condition in the copy routine.

Must begin with '/'

This reverts commit 4a0b3049.

Relevant info should be copied during txn_begin only.

Just a trial. This may not make sense if we decide to split the
meta pages into their own separate file, to allow meta traffic to
reside on a separate spindle.

Clarify prev commit, fix Doxygen comments broken by earlier changes

Don't re-use free pages so soon; that leaves us vulnerable to
DB corruption if data syncs successfully but meta doesn't.