Do not try shared lock when closing or after error.  Track file lock
state to decide.  Change meaning of *excl to reflect file lock state.

Drop the sem_unlink() error checks, which could prevent the 2nd
unlink.  Instead use O_EXCL in sem_open().

This makes "open+close the database" the API for trying to clean
away the old semaphores, if they were left behind by a previous run.

Avoid race between numreaders++ and reading numreaders at cleanup. Make
the un-mutexed reset of reader table entry, atomic: Reset mr_pid only.

Instead check mr_pid != 0 in mdb_page_alloc()'s scan for readers.
(txnid_t)-1 as "no ID"-mark avoids a check for mr_txnid != 0.
The scan can stop when seeing an old reader.

An open MDB environment does not survive or catch fork(),
so repeating getpid() was pointless.

Rename USE_POSIX_SEM to MDB_USE_POSIX_SEM.
Separate MDB_FDATASYNC from MDB_USE_POSIX_SEM.

Fix typos, error code ranges

Allow cursors on read-only txns to be reused with later txns.

Overwrites the passed in key with the DB's key

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.