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: I6b7c118e5e2fd2b92a2b168b8454fe9707325462
1 file changed