Bug#53445: Build with -Wall and fix warnings that it generates

Apart strict-aliasing warnings, fix the remaining warnings
generated by GCC 4.4.4 -Wall and -Wextra flags.

One major source of warnings was the in-house function my_bcmp
which (unconventionally) took pointers to unsigned characters
as the byte sequences to be compared. Since my_bcmp and bcmp
are deprecated functions whose only difference with memcmp is
the return value, every use of the function is replaced with
memcmp as the special return value wasn't actually being used
by any caller.

There were also various other warnings, mostly due to type
mismatches, missing return values, missing prototypes, dead
code (unreachable) and ignored return values.

include/m_string.h

@@ -33,10 +33,6 @@
 /* need by my_vsnprintf */
 #include <stdarg.h> 
 
-#ifdef _AIX
-#undef HAVE_BCMP
-#endif
-
 /*  This is needed for the definitions of bzero... on solaris */
 #if defined(HAVE_STRINGS_H)
 #include <strings.h>
@@ -60,14 +56,6 @@
 /* Unixware 7 */
 #if !defined(HAVE_BFILL)
 # define bfill(A,B,C)           memset((A),(C),(B))
-# define bmove_align(A,B,C)    memcpy((A),(B),(C))
-#endif
-
-#if !defined(HAVE_BCMP)
-# define bcopy(s, d, n)		memcpy((d), (s), (n))
-# define bcmp(A,B,C)		memcmp((A),(B),(C))
-# define bzero(A,B)		memset((A),0,(B))
-# define bmove_align(A,B,C)     memcpy((A),(B),(C))
 #endif
 
 #if defined(__cplusplus)
@@ -120,15 +108,6 @@ extern	void bfill(uchar *dst,size_t len,pchar fill);
 extern	void bzero(uchar * dst,size_t len);
 #endif
 
-#if !defined(bcmp) && !defined(HAVE_BCMP)
-extern	size_t bcmp(const uchar *s1,const uchar *s2,size_t len);
-#endif
-#ifdef HAVE_purify
-extern	size_t my_bcmp(const uchar *s1,const uchar *s2,size_t len);
-#undef bcmp
-#define bcmp(A,B,C) my_bcmp((A),(B),(C))
-#endif /* HAVE_purify */
-
 #ifndef bmove512
 extern	void bmove512(uchar *dst,const uchar *src,size_t len);
 #endif