patch 8.1.1414: alloc() returning "char_u *" causes a lot of type casts

Problem:    Alloc() returning "char_u *" causes a lot of type casts.
Solution:   Have it return "void *". (Mike Williams)  Define ALLOC_ONE() to
            check the simple allocations.
diff --git a/src/os_amiga.c b/src/os_amiga.c
index d99c198..6b8eddb 100644
--- a/src/os_amiga.c
+++ b/src/os_amiga.c
@@ -580,7 +580,7 @@
 #ifdef __amigaos4__
     fib = AllocDosObject(DOS_FIB,0);
 #else
-    fib = (struct FileInfoBlock *)alloc(sizeof(struct FileInfoBlock));
+    fib = ALLOC_ONE(struct FileInfoBlock);
 #endif
     if (fib != NULL)
     {
@@ -1448,7 +1448,7 @@
 #ifdef __amigaos4__
     Anchor = AllocDosObject(DOS_ANCHORPATH, AnchorTags);
 #else
-    Anchor = (struct AnchorPath *)alloc_clear(ANCHOR_SIZE);
+    Anchor = alloc_clear(ANCHOR_SIZE);
 #endif
     if (Anchor == NULL)
 	return 0;