Fix various clang-tidy lints

* some things that can throw are marked noexcept
  yet the linter seems to think not. Maybe they can't throw in practice.
  I would rather not have the UB possibility in pretty obvious cold
  paths.
* various default-case-missing complaints
* a fair pile of casts from integer to character, which are in fact
  deliberate.
* an instance of <https://clang.llvm.org/extra/clang-tidy/checks/bugprone/move-forwarding-reference.html>
* bugprone-not-null-terminated-result on handing a string to curl in
  chunks of bytes. our usage is fine.
* reassigning a unique_ptr by CRIMES instead of using release(), then
  using release() and ignoring the result. wild. let's use release() for
  its intended purpose.

Change-Id: Ic3e7affef12383576213a8a7c8145c27e662513d

src/libutil/source-path.cc

@@ -50,6 +50,7 @@ InputAccessor::DirEntries SourcePath::readDirectory() const
         case DT_REG: type = InputAccessor::Type::tRegular; break;
         case DT_LNK: type = InputAccessor::Type::tSymlink; break;
         case DT_DIR: type = InputAccessor::Type::tDirectory; break;
+        default: break; // unknown type
         }
         res.emplace(entry.name, type);
     }