Stdio.sendfile: Fixed handling of file length argument in nb_sendfile.

Fixes [bug 6118].
System.getgrent: The group member list can be NULL on MacOS X.

Potential fix for [bug 6187].
Fixed compat resolver fallback order.

The compat resolvers for old versions could sometimes skip
past the intermediate resolvers and go directly to the master.
Image.JPEG: Fix integer underflow. Fixes [bug 6413].
Runtime: Avoid use of negative array index in MAKE_{NAN,INF}

Apparently clang doesn't support indexing with a negative literal,
so switch the implementation to use unions instead, whenever possible.
(cherry picked from commit 6331fed0a6b5bab3b3ba7cae336b1b1b68096018)

Conflicts:

	src/port.h
(cherry picked from commit 32702d20bfaa6883210e14f5364bdcc04d4163cf)