app.c: Allow ampersands in playback lists to be escaped.

Any function or application that accepts a `&`-separated list of
filenames can now include a literal `&` in a filename by wrapping the
entire filename in single quotes, e.g.:

```
exten = _X.,n,Playback('https://example.com/sound.cgi?a=b&c=d'&hello-world)
```

Fixes #172

UpgradeNote: Ampersands in URLs passed to the `Playback()`,
`Background()`, `SpeechBackground()`, `Read()`, `Authenticate()`, or
`Queue()` applications as filename arguments can now be escaped by
single quoting the filename. Additionally, this is also possible when
using the `CONFBRIDGE` dialplan function, or configuring various
features in `confbridge.conf` and `queues.conf`.

(cherry picked from commit 33213c1979)

main/app.c

@@ -205,7 +205,7 @@ enum ast_getdata_result ast_app_getdata_terminator(struct ast_channel *c, const
 		prompt = "";
 
 	filename = ast_strdupa(prompt);
-	while ((front = strsep(&filename, "&"))) {
+	while ((front = ast_strsep(&filename, '&', AST_STRSEP_STRIP | AST_STRSEP_TRIM))) {
 		if (!ast_strlen_zero(front)) {
 			res = ast_streamfile(c, front, ast_channel_language(c));
 			if (res)