illumos gate

Userland mutexes are a bit unfortunate. By default they don't check for errors at all and even if you do enable error checking mutexes, you have to wrap everything up in VERIFY0's. Instead, we should take a leaf from the kernel and add two private symbols to libc, mutex_enter() and mutex_exit() that behave identically to their kernel compatriots. Importantly these return void, they require error checking mutexes and if we don't have what we expect, we'll explicitly panic; i.e., abort().