Attempt recovery in case of invalid next_block header field

* src/gdbmopen.c (validate_header): Return GDBM_NEED_RECOVERY
if next_block is invalid.
(_gdbm_validate_header): New function.
(gdbm_fd_open): Set need_recovery depending on return from validate_header.
(gdbm_open): Bail out on invalid value of GDBM_OPENMASK bits.
* src/proto.h (_gdbm_validate_header): New proto.
* src/recover.c (check_db): Re-validate the header.
* src/gdbmtool.c (export_handler): Fix option processing.