Add "nix verify-paths" command

Unlike "nix-store --verify-path", this command verifies signatures in
addition to store path contents, is multi-threaded (especially useful
when verifying binary caches), and has a progress indicator.

Example use:

$ nix verify-paths --store https://cache.nixos.org -r $(type -p thunderbird)
...
[17/132 checked] checking ‘/nix/store/rawakphadqrqxr6zri2rmnxh03gqkrl3-autogen-5.18.6’

src/libutil/util.cc

@@ -548,7 +548,7 @@ void writeToStderr(const string & s)
 }
 
 
-void (*_writeToStderr) (const unsigned char * buf, size_t count) = 0;
+std::function<void(const unsigned char * buf, size_t count)> _writeToStderr;
 
 
 void readFull(int fd, unsigned char * buf, size_t count)