Extract file_search.c from misc2.c

Continue to split misc2.c in many other files (see #209).

The only changed I made to the moved code was adding
`vim_free(ff_expand_buffer)` to `free_finfile()`. This is was needed
because `ff_expand_buffer` was moved from `misc2.c` to `file_search.c`.

src/eval.c

@@ -33,6 +33,7 @@
 #include "message.h"
 #include "misc1.h"
 #include "misc2.h"
+#include "file_search.h"
 #include "garray.h"
 #include "move.h"
 #include "normal.h"