build/install-sx.sh: parse user-specified options and allow --branch + --testnet.

--branch sets default branch for checkouts (useful for develop).

--testnet passes --enable-testnet to libbitcoin and libwallet.
build: just prepend to PATHs; always display where files were installed in.
build: allow --branch-keep to skip git checkouts (and other source pulls).
build: have BIN_DIR as a variable.
build: usage() exits, don't have to do it twice.