proper recovery from EINTR

[Doug Gwyn <gwyn>]

> The problem is to write two functions v_read and v_write which behave
> like the UNIX system calls read and write, except that they work "properly"
> on a slow device while (unrelated) signals are being caught.

On 4.2BSD, you could block signals during a read() or write() without
losing them.  This would be MUCH better than character-at-a-time
I/O system calls!