libstore: always unmask all signals when launching build processes

when starting builders we want the inner processes to run with a blank
slate. if some signals are masked for any reason the builder processes
may attempt to send signals to each other that are never delivered; we
avoid this by unmasking all singals. since every build tree also has a
session and process group of its own we are not in danger of sending a
signal to a builder by accident from any source, so unmasking is okay.

Change-Id: I90720ed2bd44502ffb6d2bb848c05369809abeba

lix/libstore/build/local-derivation-goal.cc

@@ -1211,7 +1211,13 @@ void LocalDerivationGoal::runChild(const Path & builder, const Strings & envStrs
 
         logger = makeSimpleLogger();
 
-        restoreSignals();
+        {
+            sigset_t set;
+            sigemptyset(&set);
+            if (sigprocmask(SIG_SETMASK, &set, nullptr)) {
+                throw SysError("failed to unmask signals");
+            }
+        }
 
         /* Put the child in a separate session (and thus a separate
            process group) so that it has no controlling terminal (meaning