ITS#7952 .gitignore mdb_dump/load
ITS#7953 .gitignore *.lo
ITS#7955, #7671 fix MDB_PREV_DUP
ITS#7956 fix compact of empty env
ITS#7841 trivial cleanup

TRIVIA - lmdb: clean testdb-dir while "make test".
ITS#7959 Check fstat return value

Signed-off-by: Jean-Christophe DUBOIS <jcd@tribudubois.net>
ITS#7961 fix txn init

More fallout from 4d02c741b120786df1b87ee9ed49c1d3f9bc7522
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().