tool_getparam: initial --json support

Adds these test cases:

 383 - simple single command line option
 384 - reading it from stdin
 385 - getting two --json options on command line
 386 - --next works after --json

Closes #8314

docs/cmdline-opts/Makefile.inc

@@ -5,7 +5,7 @@
 #                            | (__| |_| |  _ <| |___
 #                             \___|\___/|_| \_\_____|
 #
-# Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
+# Copyright (C) 1998 - 2022, Daniel Stenberg, <daniel@haxx.se>, et al.
 #
 # This software is licensed as described in the file COPYING, which
 # you should have received as part of this distribution. The terms
@@ -111,6 +111,7 @@ DPAGES = \
   interface.d \
   ipv4.d \
   ipv6.d \
+  json.d \
   junk-session-cookies.d \
   keepalive-time.d \
   key-type.d \

docs/cmdline-opts/json.d

@@ -0,0 +1,32 @@
+Long: json
+Arg: <data>
+Help: HTTP POST JSON
+Protocols: HTTP
+See-also: data-binary data-raw
+Mutexed: form head upload-file
+Category: http post upload
+Example: --json '{ "drink": "coffe" }' $URL
+Example: --json '{ "drink":' --json ' "coffe" }' $URL
+Example: --json @prepared $URL
+Example: --json @- $URL < json.txt
+Added: 7.82.0
+---
+Sends the specified JSON data in a POST request to the HTTP server. --json
+works as a shortcut for passing on these three options:
+
+ --data [arg]
+ --header "Content-Type: application/json"
+ --header "Accept: application/json"
+
+There is **no verification** that the passed in data is actual JSON or that
+the syntax is correct.
+
+If you start the data with the letter @, the rest should be a file name to
+read the data from, or a single dash (-) if you want curl to read the data
+from stdin. Posting data from a file named \&'foobar' would thus be done with
+--json @foobar and to instead read the data from stdin, use --json @-.
+
+If this option is used more than once on the same command line, the additional
+data pieces will be concatenated to the previous before sending.
+
+The headers this option sets can be overriden with --header as usual.

docs/options-in-versions

@@ -99,6 +99,7 @@
 --interface                          7.3
 --ipv4 (-4)                          7.10.8
 --ipv6 (-6)                          7.10.8
+--json                               7.82.0
 --junk-session-cookies (-j)          7.9.7
 --keepalive-time                     7.18.0
 --key                                7.9.3