ITS#7878 Replace uint32_t with unsigned in back-mdb

ITS#7878 Replace uint32_t with unsigned in back-mdb

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; adjust 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. Ensure both values are
parsed successfully before making any changes.

Fixes a compilation failure under MinGW, where stdint.h types are not
implicitly pulled in by other headers.