Fix DetectReplaceAddToList

I see two problems:
1) If allocating a newlist fails, the function returns NULL, which then
   leaks any existing list elements.
2) The code to add the new value to the list works for the first two, but
   for not the third. For example, replist=A, A->next=B, B->next=NULL, then
   adding C results in replist=A, A->next=C, C->next=NULL, B is lost.

The fix pushes new values onto the head of the list, which might not be
what is needed, but there are no comments on what the function does, so I
made an assumption.
Add a comment for DetectReplaceList

Reworded a quote in PR 742 by Regit from Inliniac to explain why adding
the head of the list (really a FIFO) is the correct behavior.
Formatting cleanup in detect-replace.c

Wrap lines longer than 80 characters
Add "static" for unit tests.
Use (void) for () for function arguments.
Add space after "while(" -> "while ("
Remove space after function names.
Put open bracket of function on a new line.