tty signal fixes

[hexagonal-sun]

Fixes: #59

• thread_group: pgid: inherit from parent
  Inherit the process-group ID from the parent. If there is no parent
  specified then the pgid is the same as the tgid.

• fd_table: don't close CLOEXEC FDs until exec
  Currently we close CLOEXEC FDs when we call clone() when making a copy
  of the file descriptor table. Defer this until the exec syscall.

  This fixes numerous bugs, namely with bash and setting the foreground
  process group for the current TTY.

• thread_group: deliver_signal: ensure signal delivery
  If all tasks within a thread group are sleeping when a signal is
  delivered, wake at least one task to action the signal.

• libkernel: error: add Interrupted
  Add the Interrupted discriminant for the KernelError enum. Also map
  it to Linux's EINTR error value.

• process: peek_signal: new
  Add a new function that allows for peeking at a task's currently pending
  signals without consuming the signal.

• process: signals: add interruptable()
  Add a new struct, InterruptableFut which allows signal
  short-circuiting logic. If a future within the kernel's syscall logic is
  wrapped in a InterruptableFut, then a wakeup with any pending signals
  causes the underlying future to be dropped and it's operation cancelled.

  Provide a InterruptResult enum to allow the caller to know whether the
  operation was interrupted and allows them to take appropriate action.
  Typically exiting with -EINTR.

  Finally, provide a blanket implementation for all futures, allowing then
  to call .interruptable() to easily wrap any future.

• usertest: make sleep interruptable()
  Allow the nanosleep family of functions to be interrupted. When an
  interruption occures, calculation the remaining duration and write that
  back to user-space.

  Add a test to ensure proper functionality into usertest.

• pipe: read: allow interruption
  Allow interrupt for read() calls on a pipe. If the syscall is
  interrupted simply return -EINVAL.

  Add a testcase to usertest for this case.

• tty: read: allow interruption
  Allow a read() on a tty to be interrupted by a syscall. This is the
  fundamental fix for making '^C' interrupt a process like cat when it's
  reading from stdin.

• sched: remove force_resched()
  This function is used to force the scheduler to reschedule another task,
  avoiding the fast-path exit. Given the sheer number of task state
  change points in the code, the fast-path exit code has become too
  brittle.

  Instead, check that the current task's state is still Running before
  taking the fast-path short-circuit. The cost of one spinlock uncontended
  lock-unlock cycle is worth the cost of avoiding many subtle scheduling
  logic bugs.

• sys_wait4: make interruptable
  Make the sys_wait4 system call interruptable via signal delivery.

  Also include some a test in usertest to ensure proper functionality.