Fix 'deadlock: trying to re-acquire self-held lock'

This was caused by derivations with 'allowSubstitutes = false'. Such
derivations will be built locally. However, if there is another
SubstitionGoal that has the output of the first derivation in its
closure, then the path will be simultaneously built and substituted.

There was a check to catch this situation (via pathIsLockedByMe()),
but it no longer worked reliably because substitutions are now done in
another thread. (Thus the comment 'It can't happen between here and
the lockPaths() call below because we're not allowing multi-threading'
was no longer valid.)

The fix is to handle the path already being locked in both
SubstitutionGoal and DerivationGoal.

src/libstore/pathlocks.hh

@@ -2,10 +2,8 @@
 
 #include "util.hh"
 
-
 namespace nix {
 
-
 /* Open (possibly create) a lock file and return the file descriptor.
    -1 is returned if create is false and the lock could not be opened
    because it doesn't exist.  Any other error throws an exception. */
@@ -18,6 +16,7 @@ enum LockType { ltRead, ltWrite, ltNone };
 
 bool lockFile(int fd, LockType lockType, bool wait);
 
+MakeError(AlreadyLocked, Error);
 
 class PathLocks
 {
@@ -38,9 +37,6 @@ public:
     void setDeletion(bool deletePaths);
 };
 
-
-// FIXME: not thread-safe!
 bool pathIsLockedByMe(const Path & path);
 
-
 }