Fixed missing NOT_AN_INTERRUPT constant in digitalPinToInterrupt()

Fixes #2379
Revert "Match return value to type in available()"

This reverts commit f40e4713542fa862d5b99b256a642e001a796988.
Added an hint for the buffer sizes.

See #2057
Fixes #2367
Updated revision log
Backported SPI Transcations from 1.5.x
Backported 'yield()' hook from 1.5.x
Backported Ethernet library from 1.5.x
Backported SD library from 1.5.x
Update revision log