Avoid possible symbol clashes with MDB-Tools libmdb

mdb_cursor_sibling() no longer pops cursor before returning.

Avoid naming conflicts with other mdb* packages. Bump version to 0.9.5

With MDB_MAPASYNC, the API provided no way to ensure full sync.

It returned 0, which is the freelist DBI and is valid for a few uses.

This allows for other flag values in the future.

Let the user specify the number of databases he will actually
use. Adding 2 for FREE_DBI and MAIN_DBI is an internal matter.

mdb_env_open() with these flags did not open a synchronizing
file descriptor.  Thus disabling them later did not work.

Drop mmap()'s MAP_FIXED flag, so it returns another address instead
of unmapping existing overlapping pages. Verify the returned address.

MDB_NOSUBDIR was == MDB_REVERSEKEY. That affected the freelist:
Env flags are stored in mm_flags alias mm_dbs[FREE_DBI].md_flags.

It stays a persistent flag, in case mdb_env_open someday wants
to pick/verify which lockfile to use with the datafile.

Catch bad flags so they will no longer make it into the data file.

Wasn't pushing the optimal half of the array onto the stack,
thus used more stack than expected -> overrun.

This reverts commit ef2092ea.

If no data was modified in the txn, mdb_open flag changes were dropped.

Could return 1 garbage record before actual data, if starting from EOF

Fix #define _GNU_SOURCE = 1 to match any command line -D_GNU_SOURCE.
.gitignore += mdb_copy.

Add flag MDB_ENV_ACTIVE instead of using the state of me_lfd.
Require MDB_RDONLY for MDB_ROFS.  Swap reader/writer mutex lock.