MINOR: sample: add "json" converter

This converter escapes string to use it as json/ascii escaped string.
It can read UTF-8 with differents behavior on errors and encode it in
json/ascii.

json([<input-code>])
  Escapes the input string and produces an ASCII ouput string ready to use as a
  JSON string. The converter tries to decode the input string according to the
  <input-code> parameter. It can be "ascii", "utf8", "utf8s", "utf8"" or
  "utf8ps". The "ascii" decoder never fails. The "utf8" decoder detects 3 types
  of errors:
   - bad UTF-8 sequence (lone continuation byte, bad number of continuation
     bytes, ...)
   - invalid range (the decoded value is within a UTF-8 prohibited range),
   - code overlong (the value is encoded with more bytes than necessary).

  The UTF-8 JSON encoding can produce a "too long value" error when the UTF-8
  character is greater than 0xffff because the JSON string escape specification
  only authorizes 4 hex digits for the value encoding. The UTF-8 decoder exists
  in 4 variants designated by a combination of two suffix letters : "p" for
  "permissive" and "s" for "silently ignore". The behaviors of the decoders
  are :
   - "ascii"  : never fails ;
   - "utf8"   : fails on any detected errors ;
   - "utf8s"  : never fails, but removes characters corresponding to errors ;
   - "utf8p"  : accepts and fixes the overlong errors, but fails on any other
                error ;
   - "utf8ps" : never fails, accepts and fixes the overlong errors, but removes
                characters corresponding to the other errors.

  This converter is particularly useful for building properly escaped JSON for
  logging to servers which consume JSON-formated traffic logs.

  Example:
     capture request header user-agent len 150
     capture request header Host len 15
     log-format {"ip":"%[src]","user-agent":"%[capture.req.hdr(1),json]"}

  Input request from client 127.0.0.1:
     GET / HTTP/1.0
     User-Agent: Very "Ugly" UA 1/2

  Output log:
     {"ip":"127.0.0.1","user-agent":"Very \"Ugly\" UA 1\/2"}
BUILD/MINOR: ssl: de-constify "ciphers" to avoid a warning on openssl-0.9.8

When building on openssl-0.9.8, since commit 23d5d37 ("MINOR: ssl: use
SSL_get_ciphers() instead of directly accessing the cipher list.") we get
the following warning :

	src/ssl_sock.c: In function 'ssl_sock_prepare_ctx':
	src/ssl_sock.c:1592: warning: passing argument 1 of 'SSL_CIPHER_description' discards qualifiers from pointer target type

This is because the openssl API has changed between 0.9.8 and 1.0.1 :

	0.9.8: char *SSL_CIPHER_description(SSL_CIPHER *,char *buf,int size);
	1.0.1: char *SSL_CIPHER_description(const SSL_CIPHER *,char *buf,int size);

So let's remove the "const" type qualifier to satisfy both versions.
Note that the fix above was backported to 1.5, so this one should as well.