Rearrange the kern_getcwd() procedure to return the base of the string

Rearrange the kern_getcwd() procedure to return the base of the string
rather then relocating the string.

Also fix two bugs: (1) the original bcopy was copying data beyond the end of
the buffer ([bp, bp+buflen] exceeds the buffer), and (2), the uap->buflen
checks must be made in __getcwd(), before the kernel tries to malloc() space.