attachInterrupt() should clear EIC INTFLAG or prior events will cause an interrupt

Consider the following pseudocode:
attachInterrupt()....enable interrupts on a pin on falling edge
...
ISR: deal with an interrupt and clear INTFLAG
...
detachInterrupt()...disable interrupt on the pin
...
there is a falling edge on the pin while interrupts are disabled
...
attachInterrupt()...enable interrupts again

An interrupt will be generated, even if the interrupt flag was cleared in the ISR!

This doesn't happen if the interrupt flag is cleared immediately before attachInterrupt() is called:
e.g. for EXTINT[6]
`REG_EIC_INTFLAG = EIC_INTFLAG_EXTINT6; // Clear the EXTINT6 flag`

It seems to make sense to modify attachInterrupt() to clear the interrupt flag before enabling the interrupt, and/or add a function to clear the interrupt that doesn't require the user to go into the spec sheet and Google and figure everything out by themselves.

Start at the attachInterrupt() implementation and review how the SAMD21 EIC interrupt is enabled, using the EXTINT[6] example and EIC_INTFLAG behavior described in the issue. Reproduce the detach/edge/reattach sequence and verify that reattaching does not generate an interrupt from an event that occurred while interrupts were disabled.

Written by the indexing model from the issue text.