ITS#7878 Replace uint32_t with unsigned in back-mdb

This fixes a compilation failure under MinGW, where stdint.h types are not implicitly pulled in by other headers. The quick fix is to #include <stdint.h>, however upon inspecting the types and their uses, it looked like using the fixed-size types is actually unnecessary. Tested on Linux (Debian 9) and MinGW (MSYS2).

init.c: align mi_dbenv_flags and flags with mdb_dbi_open, which declares flags as unsigned int.

search.c: align mi_rtxn_size with ARG_UINT, which assigns an unsigned int; also change ww_ctx.nentries to silence a warning about signed/unsigned comparison.

config.c: parse checkpoint config more carefully. Reject negative or unreasonably large values for kbytes and minutes (previously, negative or large values were just reinterpreted as uint32_t). Ensure both values are parsed successfully before making any changes.