Change all primitives that use the `stat' and `lstat' system calls to

Change all primitives that use the `stat' and `lstat' system calls to
check for EINTR, and to signal system-call errors if the file's
existence cannot be determined.  Change the FILE-EXISTS? primitive to
return zero (instead of #F, as previously) if the file being tested is
a symbolic link but it doesn't point at anything.