improve manifest filenames

[behindtext]

manifest filenames are currently of following form:

manifest-vX.Y.Z.txt is for the command line tools
manifest--vX.Y.Z.txt is for all other repos (dcrinstall, decrediton, paymetheus, etc)

to improve the ease with which these files can be found in file browsers and to avoid filename conflicts with other projects, i propose changing the format to roughly:

decredcli-manifest-vX.Y.Z.txt for command line tools
-manifest-vX.Y.Z.txt for other repos