Rather than invoking fork/execve of dc(1) on a pipe, compile in the dc(1)

Rather than invoking fork/execve of dc(1) on a pipe, compile in the dc(1)
code directly and use it as a subfunction.  This refactoring allows use of
pledge "stdio rpath proc tty" in the main bc(1) process before fork, pledge
"stdio rpath tty" after fork, and fully reduced to "stdio" in the dc(1)
child.

This requires two recent to the kernel code (allowing sigsuspend(),
and kill() self as pid 0).
ok otto