To allow reloading of custom-sorted DBs from mdb_dump

On DUPSORT DBs, must initialize xcursor regardless of whether
caller requested its data. Also in cursor_prev must check whether
cursor index is still within range before using it.

FreeBSD 11 supports robust process-shared POSIX mutexes,
but requires them to be explicitly destroyed before munmap

mdb_load wasn't properly inserting escaped backslashes into the data.
mdb_dump wasn't escaping backslashes when generating printable output.

Kris Zyp authored Apr 15, 2019 and Howard Chu committed Apr 16, 2019

Broken in ITS#8756

Kris Zyp authored Feb 06, 2019 and Howard Chu committed Feb 10, 2019

CreateFileMapping already takes care of it.

Bump up number of keys for which we use fine-grained splitpoint search

Unnecessary since these are DUPs, the key will always be the same

Avoid stomping on flags from 1st readhr invocation