Use posix_spawn to run the pager

In low memory environments, "nix-env -qa" failed because the fork to
run the pager hit the kernel's overcommit limits. Using posix_spawn
gets around this. (Actually, you have to use posix_spawn with the
undocumented POSIX_SPAWN_USEVFORK flag, otherwise it just uses
fork/exec...)

src/libutil/util.cc

@@ -45,8 +45,14 @@ BaseError & BaseError::addPrefix(const FormatOrString & fs)
 
 
 SysError::SysError(const FormatOrString & fs)
-    : Error(format("%1%: %2%") % fs.s % strerror(errno))
-    , errNo(errno)
+    : SysError(errno, fs)
+{
+}
+
+
+SysError::SysError(int errNo, const FormatOrString & fs)
+    : Error(format("%1%: %2%") % fs.s % strerror(errNo))
+    , errNo(errNo)
 {
 }