rdmd <dmd flags> --eval

Spin-off from https://github.com/D-Programming-Language/tools/pull/140

fixes https://issues.dlang.org/show_bug.cgi?id=13344  - rdmd --eval ignores flags

Also disallows mixing of `--eval`, `--loop` and program files. Before, `--loop` would win over `--eval` would win over program files, with the losers being silently ignored.