Change default mem init behavior

Init malloc'd memory by default, use #MDB_NOMEMINIT to disable.
Bump version to 0.9.10
Clarify WRITEMAP / NOMEMINIT
Windows/Win64/MSVC tweaks

Silence warnings, fix broken MinGW64 definitions, etc.
Bump to 0.9.10
Doc tweak

Measured perf cost of meminit was 7% worst case on an Intel Core2
system. YMMV.