CLEANUP: assorted typo fixes in the code and comments

This is 31st iteration of typo fixes
diff --git a/src/ncbuf.c b/src/ncbuf.c
index d2e17fe..1202939 100644
--- a/src/ncbuf.c
+++ b/src/ncbuf.c
@@ -506,7 +506,7 @@
 	return !ncb_blk_is_last(buf, gap);
 }
 
-/* Returns the number of bytes of data avaiable in <buf> starting at offset
+/* Returns the number of bytes of data available in <buf> starting at offset
  * <off> until the next gap or the buffer end. The counted data may wrapped if
  * the buffer storage is not aligned.
  */
@@ -589,7 +589,7 @@
 		ncb_sz_t done;
 
 		/* retrieve the next block. This is necessary to do this
-		 * before overwritting a gap.
+		 * before overwriting a gap.
 		 */
 		next = ncb_blk_next(buf, blk);