Fix building libbinder against musl

musl libc uses the posix definition of msg_iovlen as int.  Change the
niovs argument to an int, make sure it is not negative, and cast it to
size_t if necessary.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: I6ff9206e9e7396f2f89622735f790b4fac18f76c