Spawn external programs asynchronously

Drawback is that we can't check the exitcode anymore.
But we were unsure why/when we need this, see:
https://github.com/profanity-im/profanity/pull/1760/files#r980868708

Fixes https://github.com/profanity-im/profanity/issues/1759

src/tools/aesgcm_download.c

@@ -155,7 +155,7 @@ aesgcm_file_get(void* userdata)
                                                  aesgcm_dl->filename);
 
         // TODO: Log the error.
-        if (!call_external(argv, NULL, NULL)) {
+        if (!call_external(argv)) {
             http_print_transfer_update(aesgcm_dl->window, aesgcm_dl->url,
                                        "Downloading '%s' failed: Unable to call "
                                        "command '%s' with file at '%s' (%s).",

src/tools/http_download.c

@@ -202,7 +202,7 @@ http_file_get(void* userdata)
                                                  download->filename);
 
         // TODO: Log the error.
-        if (!call_external(argv, NULL, NULL)) {
+        if (!call_external(argv)) {
             http_print_transfer_update(download->window, download->url,
                                        "Downloading '%s' failed: Unable to call "
                                        "command '%s' with file at '%s' (%s).",