add new --no-eta, better OOM handling, other misc

 * new --no-eta option: completely disables eta calculations and
pre-counting of passwords
 * new out-of-memory handler which frees up memory, if possible, so a
final autosave might succeed, and also prints a suggestion
 * --skip now affects --listpass
permit using stdin for tokenlist or passwordlist

specify "-" for stdin, e.g.: "--passwordlist -"