Fix #695: Lazy load all symbols required by the compiler

Invoking scalajsc without any files loaded the compiler (including
Scala.js plugin) without a run or a compilation unit. This in turn
caused the symbol loads to crash since the Scala library wasn't
loaded.
Fix #721: Make scalajsc run in standard shell
Fix #700: Preserve executable flag on CLI runners
Merge pull request #722 from gzm0/cli-fixes

Cli fixes