Allow a POSIX shared memory object that is opened for read but not for write to nonetheless be mapped PROT_WRITE and MAP_PRIVATE, i.e., copy-on-write.
Allow a POSIX shared memory object that is opened for read but not for
write to nonetheless be mapped PROT_WRITE and MAP_PRIVATE, i.e.,
copy-on-write.

(This is a regression in the new implementation of POSIX shared memory
objects that is used by HEAD and RELENG_8.  This bug does not exist in
RELENG_7's user-level, file-based implementation.)

PR:		150260
MFC after:	3 weeks

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