Force sys_close not to restart by returning ERESTART.

Print a diagnostic message if we ever get ERESTART out of fd_close
and convert it to EINTR instead.

Even if fd_close fails, it has already closed the file descriptor, so
restarting the system call is a mistake, with dangerous consequences
for multithreaded programs.

Should probably turn the message into a kassert eventually, and maybe
add one deeper in fd_close in order to more easily debug it before
all the data structures are destroyed.

(riastradh)