perfuse_bufvar_from_env
opt = perfuse_bufvar_from_env("PERFUSE_BUFSIZE",
opt = perfuse_bufvar_from_env("PERFUSE_BUFSIZE",
uint32_t perfuse_bufvar_from_env(const char *, uint32_t);
opt = perfuse_bufvar_from_env("PERFUSE_BUFSIZE", 16 * FUSE_BUFSIZE);