Make different threads use different FILE*'s

Make different threads use different FILE*'s

read_wim_resource() was actually not thread safe because it expects to use the
FILE * WIMStruct.fp.  This commit introduces a table of FILE *'s for each
WIMStruct that are only used if a special flag
WIMLIB_RESOURCE_FLAG_MULTITHREADED is passed to read_wim_resource().