Permit private writable mappings of read-only /dev/zero descriptors.

[jhb]

Summary

The changes that introduced fo_mmap() treated all character device
mappings as if MAP_SHARED was always present since in general MAP_PRIVATE
is not permitted for character devices. However, there is one exception
in that MAP_PRIVATE mappings are permitted for /dev/zero.

Only require a writable file descriptor (FWRITE) for shared, writable
mappings of character devices. vm_mmap_cdev() will reject any private
mappings for other devices.

Test Plan

• Open /dev/zero with O_RDONLY and try to map it with PROT_WRITE and MAP_PRIVATE.
• Verified that a MAP_SHARED mapping with O_RDONLY still fails.

(These tests will end up in tests/sys/vm/mmap_test.c eventually, but
that has some other pending changes and I don't want to hold up this
fix for those.)