nvram_by_symbol
extern char *nvram_by_symbol(char *); /* machdep.c */
value = nvram_by_symbol("boot_unit");
value = nvram_by_symbol("boot_partition");
char *nvram_by_symbol(char *);
static const char *nvram_by_symbol(char *);
nvv = nvram_by_symbol("boot_unit");
nvv = nvram_by_symbol("boot_partition");
nvv = nvram_by_symbol("boot_device");