Moving headers outside of overflow page.

Separate 32/64 dependency from rpage buffer mechanism

db_mid2l_insert(): Move assert to mdb.c.
mdb_cursor_set(): Previous assert cannot fail now.
mdb_cursor_put(): Check mc/key and return EINVAL.
mdb_cursor_dbi(): No error return, so just segfault if cursor==NULL.

Drop unused computed/assigned values.
mdb_page_alloc(): Make "mop_len>0 implies mop!=NULL" provable.

Happened since sqlightning #includes mdb.c, midl.c.

Page leak, mdb_page_alloc(). On error, don't shorten me_pghead.
Memleak, mdb_ovpage_free(). Free page or keep it in dirty_list.
Bad MIDL, mdb_midl_need(). Fix midl[-1] (allocated size).

Grow midls earlier in order to catch errors earlier.  Use
mdb_midl_need() instead of mdb_midl_grow(), then mdb_midl_xappend()
needs no error checks. Factor out mdb_midl_append_range().

MDB_env.me_pghead: Don't free it when empty. mdb_ovpage_free()
needs it, but cannot allocate it.
mdb_midl_alloc(): Fill in length=0.
mdb_page_alloc(): Also Skip freeDB if txnid<3, instead of <4,
and consistently DPRINTF consumed IDLs.

Forgot #include <errno.h>

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

Prefix ID, NOID, ID2, IDL, ID2L with 'MDB_'.
Also drop bdb from file description.

Microoptimize IDL search.
Use RANGE_<FIRST/LAST> when IDL is known to be a range.

Allow MIDLs to grow arbitrarily, to allow unlimited number of
pages to be freed in one txn.

Parenthesize.  Rename GetPageSize -> GET_PAGESIZE since it does not
behave like a function (it has a non-pointer output argument).