(trunk) replace the tr_metainfo_builder.isSingleFile and tr_info.isMultifile fields...

(trunk) replace the tr_metainfo_builder.isSingleFile and tr_info.isMultifile fields an 'isFolder' bool in both structs.

This makes the variable naming more uniform. It also clarifies the information we're really trying to convey -- previously, isSingleFile was false whenever the torrent held a directory tree, even if there was only a single file in the tree.

Sync the Mac OS X client's use to match the libtransmission variable names.