ITS#7959 fix prev commit

fstat returns -1, not an errno. Move code into its own function
for reuse.
ITS#7969 LMDB: volatile & __synchronize().

Globally shared fields of meta-data are not 'volatile'.
ITS#7970 LMDB: Critical Heisenbug

Inconsistent reading & SIGSEGV due to the race condition.
ITS#7971 LMDB: clarification in mdb_txn_renew0().