Var: added a $dollar variable that translates to a '$'.

Allow $dollar (or ${dollar}) to translate to a literal $ to allow
support for sub-delimiters in URIs.

It is possible to have URLs like

  https://example.com/path/15$1588/9925$2976.html

and thus it would be useful to be able to specify them in various bits
of the unit config such as the location setting.

However this hadn't been possible due to $ being used to denote
variables for substitution. E.g $host.

As was noted in the below GitHub issue it was suggested by @VBart to
use $sign to represent a literal $, however I feel $dollar is more
appropriate so we have a variable named after the thing it represents,
also @tippexs found[0] that &dollar is used in HTML to represent a $, so
there is some somewhat related precedent.

(The other idea to use $$ was rejected in my original pull-request[1]
 for this issue.)

This means the above URL could be specified as

  https://example.com/path/15${dollar}1588/9925${dollar}2976.html

in the unit config.

This is done by adding a variable called 'dollar' which is loaded into
the variables hash table which translates into a literal $.

This is then handled in nxt_var_next_part() where variables are parsed
for lookup and $dollar is set for substitution by a literal '$'. Actual
variable substitution happens in nxt_var_query_finish().

[0]: https://github.com/nginx/unit/pull/693#issuecomment-1130412323
[1]: https://github.com/nginx/unit/pull/693

Closes: https://github.com/nginx/unit/issues/675

docs/changes.xml

@@ -62,6 +62,13 @@ increased the applications' startup timeout.
 </para>
 </change>
 
+<change type="feature">
+<para>
+added a new variable, $dollar, that translates to a literal "$" during
+variable substitution.
+</para>
+</change>
+
 </changes>
 
 

src/nxt_http_variables.c

@@ -7,6 +7,8 @@
 #include <nxt_http.h>
 
 
+static nxt_int_t nxt_http_var_dollar(nxt_task_t *task, nxt_str_t *str,
+    void *ctx, uint16_t field);
 static nxt_int_t nxt_http_var_method(nxt_task_t *task, nxt_str_t *str,
     void *ctx, uint16_t field);
 static nxt_int_t nxt_http_var_request_uri(nxt_task_t *task, nxt_str_t *str,
@@ -41,6 +43,9 @@ static nxt_int_t nxt_http_var_cookie(nxt_task_t *task, nxt_str_t *str,
 
 static nxt_var_decl_t  nxt_http_vars[] = {
     {
+        .name = nxt_string("dollar"),
+        .handler = nxt_http_var_dollar,
+    }, {
         .name = nxt_string("method"),
         .handler = nxt_http_var_method,
     }, {
@@ -96,6 +101,15 @@ nxt_http_register_variables(void)
 }
 
 
+static nxt_int_t
+nxt_http_var_dollar(nxt_task_t *task, nxt_str_t *str, void *ctx, uint16_t field)
+{
+    nxt_str_set(str, "$");
+
+    return NXT_OK;
+}
+
+
 static nxt_int_t
 nxt_http_var_method(nxt_task_t *task, nxt_str_t *str, void *ctx, uint16_t field)
 {