Replace WIMLIB_OPEN_FLAG_SPLIT_OK with WIMLIB_OPEN_FLAG_ERROR_IF_SPLIT

Having to specify a special flag to open split WIM parts is no longer very
useful, since error codes will be automatically be issued when needed metadata
or file resources are not found, and this flag wouldn't affect delta WIMs which
resemble split WIMs anyway, and clients could just check themselves whether a
WIM is split.  Therefore, make the default be like WIMLIB_OPEN_FLAG_SPLIT_OK,
but for completeness offer a flag WIMLIB_OPEN_FLAG_ERROR_IF_SPLIT to get
behavior like the old default.

Also updated the documentation in wimlib.h.