client - Build the ncurses/readline full featured CLI
custom-cflags - Build with user-specified CFLAGS (unsupported)
debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
filecaps - Use Linux file capabilities to control privilege rather than set*id (this is orthogonal to USE=caps which uses capabilities at runtime e.g. libcap)
libssh - Enables net-libs/libssh binding, mendatory for RPKI support
