shared/runtime/pyexec: Cleanup EXEC_FLAG flag constants.
shared/runtime/pyexec: Cleanup EXEC_FLAG flag constants.

- Cleanup pyexec flags definitions so it's clear they are different.
- Use mp_uint_t for exec_flags because it should be unsigned.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython