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/x509.c b/library/x509.c
index fad390d..4df542e 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -661,7 +661,7 @@
 
 /*
  * X.509 Extensions (No parsing of extensions, pointer should
- * be either manually updated or extensions should be parsed!
+ * be either manually updated or extensions should be parsed!)
  */
 int mbedtls_x509_get_ext( unsigned char **p, const unsigned char *end,
                   mbedtls_x509_buf *ext, int tag )