all: Add .frequency read-only property for busio.SPI to return actual frequency. Fix esp8266/posix_helpers.c, which was not up to date for the new long-lived/short-lived heap allocation scheme.
POSIX requires malloc(), etc. to set ENOMEM on the failure, and e.g. BerkeleyDB relies on this: http://pubs.opengroup.org/onlinepubs/9699919799/functions/malloc.html This should fix confusing OSError exceptions with 0 error code when working with btree module.
As it's used by BerkeleyDB, etc.