Fix spelling errors

The other spelling errors come from external projects (doxygen and
pkg-config), which are being dealt upstream.

Warned-by: codespell

ChangeLog.old

@@ -3393,7 +3393,7 @@
 
 2008-01-22  Guillem Jover  <guillem@debian.org>
 
-	* lib/dpkg.h [!offsetof] (offsetof): New compatibilty macro.
+	* lib/dpkg.h [!offsetof] (offsetof): New compatibility macro.
 	* lib/parsedump.h (PKGIFPOFF): Use offsetof instead of ad-hoc
 	calculation.
 	(FILEFOFF): Likewise.

lib/compat/obstack.h

@@ -127,7 +127,7 @@ extern "C" {
 
 #define __BPTR_ALIGN(B, P, A) ((B) + (((P) - (B) + (A)) & ~(A)))
 
-/* Similiar to _BPTR_ALIGN (B, P, A), except optimize the common case
+/* Similar to _BPTR_ALIGN (B, P, A), except optimize the common case
    where pointers can be converted to integers, aligned as integers,
    and converted back again.  If PTR_INT_TYPE is narrower than a
    pointer (e.g., the AS/400), play it safe and compute the alignment