Fix ls -d to work like GNU coretools ls
Unset placement_address after it should've been used for the last time
Start work on userspace symbols, for backtraces
Plenty of work on backtraces.

Userspace addresses now (soon) have symbols, so we can use that information
in backtraces for e.g. kernel panics and memory leaks!
Declare asm functions as functions -- backtraces are now 100% correct!
Clean up backtrace.c
Finalize backtrace work with some optimization and cleaning.

Instead of calling kmalloc twice per symbol (once for the struct symbol, once
for strdup:ing the name), call kmalloc twice TOTAL (once for an array of struct symbol
objects, and once or the entire string table).
Clone symbols in fork() and free them when tasks exit