bionic: prevent root processes from calling __system_property_add

bionic: prevent root processes from calling __system_property_add

If a root process other than init calls __system_property_add, which
it should never do, it will break the design assumption that there is
only one mutator.

Pass O_EXCL to open() in map_prop_region_rw to ensure that only one
process ever has the property pages open for write.

Change-Id: I8233bfe0beaa40a5003ad53b98c661536b15f6b7