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

library/ssl_tls.c

diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 84a04ae..121c135 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3428,7 +3428,7 @@
 
     if( ret == MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED )
     {
-        /* Dont check write errors as we can't do anything here.
+        /* Don't check write errors as we can't do anything here.
          * If the error is permanent we'll catch it later,
          * if it's not, then hopefully it'll work next time. */
         (void) ssl->f_send( ssl->p_bio, ssl->out_buf, len );
@@ -6006,8 +6006,9 @@
     const char **p;
 
     /*
-     * "Empty strings MUST NOT be included and byte strings MUST NOT be
-     * truncated". Check lengths now rather than later.
+     * RFC 7301 3.1: "Empty strings MUST NOT be included and byte strings
+     * MUST NOT be truncated."
+     * We check lengths now rather than later.
      */
     tot_len = 0;
     for( p = protos; *p != NULL; p++ )
@@ -7585,7 +7586,7 @@
  * and, for DTLS, to/from TLS equivalent.
  *
  * For TLS this is the identity.
- * For DTLS, use one complement (v -> 255 - v, and then map as follows:
+ * For DTLS, use 1's complement (v -> 255 - v, and then map as follows:
  * 1.0 <-> 3.2      (DTLS 1.0 is based on TLS 1.1)
  * 1.x <-> 3.x+1    for x != 0 (DTLS 1.2 based on TLS 1.2)
  */