tool: add "variable" support

Add support for command line variables. Set variables with --variable
name=content or --variable name@file (where "file" can be stdin if set
to a single dash (-)).

Variable content is expanded in option parameters using "{{name}}"
(without the quotes) if the option name is prefixed with
"--expand-". This gets the contents of the variable "name" inserted, or
a blank if the name does not exist as a variable. Insert "{{" verbatim
in the string by prefixing it with a backslash, like "\\{{".

Import an environment variable with --variable %name. It makes curl exit
with an error if the environment variable is not set. It can also rather
get a default value if the variable does not exist, using =content or
@file like shown above.

Example: get the USER environment variable into the URL:

 --variable %USER
 --expand-url = "https://example.com/api/{{USER}}/method"

When expanding variables, curl supports a set of functions that can make
the variable contents more convenient to use. It can trim leading and
trailing white space with "trim", output the contents as a JSON quoted
string with "json", URL encode it with "url" and base 64 encode it with
"b64". To apply functions to a variable expansion, add them colon
separated to the right side of the variable. They are then performed in
a left to right order.

Example: get the contents of a file called $HOME/.secret into a variable
called "fix". Make sure that the content is trimmed and percent-encoded
sent as POST data:

  --variable %HOME=/home/default
  --expand-variable fix@{{HOME}}/.secret
  --expand-data "{{fix:trim:url}}"
  https://example.com/

Documented. Many new test cases.

Co-brainstormed-by: Emanuele Torre
Assisted-by: Jat Satiro
Closes #11346

tests/data/test451

@@ -0,0 +1,59 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+variables
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data crlf="yes">
+HTTP/1.1 200 OK
+Date: Tue, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+ETag: "21025-dc7-39462498"
+Accept-Ranges: bytes
+Content-Length: 6
+Connection: close
+Content-Type: text/html
+Funny-head: yesyes
+
+-foo-
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+<name>
+Variable from file that is JSON and URL encoded (with null byte)
+</name>
+<file name="%LOGDIR/junk">
+%hex[%01%02%03%00%04%05%06]hex%
+</file>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER --variable what@%LOGDIR/junk --variable second=hello --variable second=again --expand-data "--{{what:trim:json}}22{{none}}--{{second}}{{what:trim:url}}"
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol crlf="yes" nonewline="yes">
+POST /%TESTNUMBER HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+User-Agent: curl/%VERSION
+Accept: */*
+Content-Length: 74
+Content-Type: application/x-www-form-urlencoded
+
+--\u0001\u0002\u0003\u0000\u0004\u0005\u000622--again%01%02%03%00%04%05%06
+</protocol>
+</verify>
+</testcase>