Clarify Comments and Fix Typos (#651)

Fixes many typos, and errors in comments.

* Clarifies many comments
* Grammar correction in config.pl help text
* Removed comment about MBEDTLS_X509_EXT_NS_CERT_TYPE.
* Comment typo fix (Dont => Don't)
* Comment typo fix (assure => ensure)
* Comment typo fix (byes => bytes)
* Added citation for quoted standard
* Comment typo fix (one complement => 1's complement)

The is some debate about whether to prefer "one's complement",  "ones'
complement", or "1's complement".  The more recent RFCs related to TLS
(RFC 6347,  RFC 4347, etc) use " 1's complement", so I followed that
convention.

* Added missing ")" in comment
* Comment alignment
* Incorrect comment after #endif
diff --git a/library/cmac.c b/library/cmac.c
index ee2fe05..04aca7c 100644
--- a/library/cmac.c
+++ b/library/cmac.c
@@ -62,7 +62,7 @@
 #if defined(MBEDTLS_SELF_TEST)
 #include <stdio.h>
 #define mbedtls_printf     printf
-#endif /* MBEDTLS_SELF_TEST && MBEDTLS_AES_C || MBEDTLS_DES_C */
+#endif /* MBEDTLS_SELF_TEST */
 #endif /* MBEDTLS_PLATFORM_C */
 
 /* Implementation that should never be optimized out by the compiler */
@@ -80,7 +80,7 @@
  *   with R_64 = 0x1B and  R_128 = 0x87
  *
  * Input and output MUST NOT point to the same buffer
- * Block size must be 8 byes or 16 bytes - the block sizes for DES and AES.
+ * Block size must be 8 bytes or 16 bytes - the block sizes for DES and AES.
  */
 static int cmac_multiply_by_u( unsigned char *output,
                                const unsigned char *input,