libc: __system_property_set uses writev to write atomically

__system_property_set sometimes produces broken_pipe error
when trying to write a property.

This change improves error messages and uses writev() instead
of sequence of send() calls.

Bug: http://b/35381074
Test: bionic-unit-tests --gtest_filter=prop*
Change-Id: I7a5b169c015db4e6b720370e58662de8206d1086