Fix a variety of bugs found in dllist.c while addressing Issue #1559.

[jmpenn]

• Added More descriptive error messages.
• Wrote google-test based unit tests for the DLL List library.
• Deleted old, incomplete test program.
• Added new function DLL_ListContainsPos which determines whether the given list contains the node at the given pos.
• DLLFind bug: Added check to determine whether a compare function has been specified, and emit an error message if it hasn’t.
• DLL_FindIndex bug: Corrected bounds checking.
• DLL_GetAt bug: Added check to ensure that the specified pos is actually in the given list.
• DLL_SetAt bug: Added check to ensure that the specified pos is actually in the given list.
• DLL_RemoveAt: corrected logic mistake in NULL ptr check. Added check to ensure that the specified pos is actually in the given list.
• DLL_InsertBefore bug: element count not correctly updated.
• DLL_InsertAfter: next and prev ptrs not correctly updated, which corrupted the list.
• DLL_GetNext bug: logic error in NULL ptr checks. Added check to ensure that the specified pos is actually in the given list.
• DLL_GetPrev bug: logic error in NULL ptr checks. Added check to ensure that the specified pos is actually in the given list.
• DLL_AddHead: Fixed NULL check logic error.
• DLL_AddTail: Fixed NULL check logic error.

[coveralls]

coverage: 55.756% (-0.003%) from 55.759% when pulling 0f357821e9944327a2c4c703156269c91c055a8f on Issue_1559 into 63093ee6059dc764005ac21c1fc1b1bf2d6b5237 on master.

[sharmeye]

I can think of at least one person who will be very happy with this update. Everything looks good to me, and i like there's a unit test for this library now.

[hchen99]

Really like those descriptive error messages