[TRint] Use `TCling::GetMore()` to tell if we are waiting for more input.
[TRint] Use `TCling::GetMore()` to tell if we are waiting for more input.

`TRint::ProcessLineNr` used the output parameter `error` to tell if the
collected input was incomplete (i.e., it contains unbalanced braces or the
last line was terminated by a `\` character).

While this works most of the time, it fails in the following case:
```
root[] gROOT->ProcessLine("fprintf(stdout,\"the inner is printing\"\n");
```
In this case, there are two nested `ProcessLine()` calls; the inner-most has
incomplete input (missing a `)`), but the `error` output parameter is ignored.
The outer-most call, however, has complete input, which is what was seen by
TRint.
This created inconsistency between the collected input in cling's InputValidator
(which was incomplete) and the ROOT prompt string, that was not showing
`root (cont'ed, cancel with .@) []`.

Fixes ROOT-5219.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root