Handle magic descriptions like "PHP script, ASCII text".

libmagic version 5.11 and possibly other versions return descriptions in the format `<language> script, ASCII text (executable)?`. For example:

    PHP script, ASCII text
    Python script, ASCII text executable
    Ruby script, ASCII text executable 

This was causing test failures such as

    run_tests: test/unit/detector_test.h:119: test_detector_detect_polyglot: Assertion `lang' failed.

The proposed PR fixes the issue by handling the new description format in `magic_parse`.