MFC r305855, r306297, r306300, r306312-r306313

When MAKEOBJDIRPREFIX points to a case-insensitive file system, the
build can break when different source files create the same object
files (case-insensitivity speaking). This is the case for object
files compiled with -fpic and shared libraries. The former uses
an extension of ".So", and the latter an extension ".so". Rename
shared object files from *.So to *.pico to match what NetBSD does.

Also:
o Compile _Exit.c as C99_Exit.c, as it conflicts with _exit.s
o Add entry to UPDATING
o Document .pico extension