Disable job control inside command substitutions

This disables job control inside command substitutions. Prior to this
change, a cmdsub might get its own process group. This caused it to fail
to cancel loops properly. For example:

    while true ; echo (sleep 5) ; end

could not be control-C cancelled, because the signal would go to sleep,
and so the loop would continue on. The simplest way to fix this is to
match other shells and not use job control in cmdsubs.

Related is #1362

src/parser.cpp

@@ -398,6 +398,18 @@ bool parser_t::is_breakpoint() const {
     return false;
 }
 
+bool parser_t::is_command_substitution() const {
+    for (const auto &b : block_list) {
+        if (b.type() == block_type_t::subst) {
+            return true;
+        } else if (b.type() == block_type_t::source) {
+            // If a function sources a file, don't descend further.
+            break;
+        }
+    }
+    return false;
+}
+
 maybe_t<wcstring> parser_t::get_function_name(int level) {
     if (level == 0) {
         // Return the function name for the level preceding the most recent breakpoint. If there