Add the ability to disable deprecation warnings with a cmd line switch

The `--depwarn={yes|no}` flag turns on/off method warnings and deprecated syntax warnings
in the parser.

(unexported) `compileropts()` method added to retrieve the
compileropts struct from libjulia.

(unexported) `syntax_deprecation_warnings(::Bool)` method added to
turn off syntax deprecation warnings in the parser at runtime.
the method returns the previous syntax deprecation warning state.

add NEWS entry and update docs for command line switches
Merge pull request #9294 from JuliaLang/jcb/nodepwarn

RFC: add the ability to disable syntax deprecation warnings