Replace fmemopen.

A new implementation starting from the FreeBSD fmemopen rather than the
OpenBSD one we used to use.

The tests were arrived at by translating each sentence in
http://pubs.opengroup.org/onlinepubs/9699919799/functions/fmemopen.html,
plus http://man7.org/linux/man-pages/man3/fmemopen.3.html for historical
GNU bugs.

Bug: http://b/31304889
Test: ran tests
Change-Id: Id8b168c9ecde638e9cdedbc3b8a0982fc83c7048