Signer infrastructure: Prep for #9076

This sets up infrastructure in libutil to allow for signing other than
by a secret key in memory. #9076 uses this to implement remote signing.

(Split from that PR to allow reviewing in smaller chunks.)

Co-Authored-By: Raito Bezarius <masterancpp@gmail.com>

src/libutil/util.cc

@@ -7,6 +7,7 @@
 #include <grp.h>
 #include <regex>
 
+#include <sodium.h>
 
 namespace nix {
 
@@ -28,6 +29,9 @@ void initLibUtil() {
     }
     // This is not actually the main point of this check, but let's make sure anyway:
     assert(caught);
+
+    if (sodium_init() == -1)
+        throw Error("could not initialise libsodium");
 }
 
 //////////////////////////////////////////////////////////////////////