curl/var: change byte offset syntax for assignments

Follow-up to 40c264db61 after discussions on IRC.

The new style is

  name[0-99]=contents

and

  name[0-99]@filename

A) This does not cause the same problems with old curl versions trying
the new syntax as this way will cause old curls just fail with syntax
error and not risk using the wrong file.

B) Adds the same byte range support for "normal" assigns, which the
previous syntax did not. Thus lets a user get a partial content of a
variable etc.

Added test 790 and 791 to verify non-file assigns with ranges.

Closes #15862

tests/data/test790

@@ -0,0 +1,56 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+--variable
+</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 with a byte range using plain assign
+</name>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER --variable "name[5-9]=0123456789abcdef" --expand-data '{{name}}'
+</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: 5
+Content-Type: application/x-www-form-urlencoded
+
+56789
+</protocol>
+</verify>
+</testcase>