fix crash in Posix_getaddrinfo for ProGuarded Android build

Posix_getaddrinfo needs to access fields in libcore.io.StructAddrinfo
via JNI, so we tell ProGuard to preserve them.

This commit also includes a minor indentation tweek in README.md and
removes -fno-rtti from lzma-build-cflags to avoid a warning from GCC.