test: self: reduce n_forks in many_fork_sequence

This lets the test complete in less than three seconds on a 2.4 gigahertz Core
2, whereas 1024 wouldn't.
space: forbid SpaceControl on kernel_space

Trivially true, and required to add a redirection-related field to thread->u1.
ipc: allow seg=0 in copy_interspace_stritem()

A similar restriction was lifted in copy_intraspace_stritem() recently.

Proof: min [PAGE_SIZE - (x & PAGE_MASK), d_len - d_offs, s_len - s_offs] will
be zero iff either of the last two terms is zero. Since the memcpy call cannot
return a fault when seg=0, neither case will go undiscovered due to
non-incrementing of s_off and d_off. Thus the loop will terminate even if
seg=0, and it follows that this need not be asserted against.