Simplify recent changes a bit.
Fix robust mutexes - repair mti_txnid.

Broken by e3b6c359a935283c21e1fd7d03b790d87d53b933, if
commit() crashes in write_meta before setting mti_txnid.
Clean up SysV semaphore code.

Change MDB_LOCK_FORMAT to differ from Posix semaphores/mutexes.
Do not use array/member length 0 for MDB_txninfo.mt2.
Fix comments
mdb_strerror: Silence warning @ Windows
fix excessive space (likely a typo) for single write txn.