diff: fix incorrectly displaying files as duplicates

When diff hits certain access errors, function diffreg() shows the error
message, and then returns to the calling function, which calls
print_status() with the return value.

However, in these cases, the return value isn't changed from the initial
default value of D_SAME.

Normally, print_status() with a value of D_SAME does nothing, so this
works out ok, however, if the "-s" flag is set, a message is displayed
showing identicality:

case D_SAME:

if (sflag)
        printf("Files %s%s and %s%s are identical\n",                                                                                                       path1, entry, path2, entry);
break;

This then produces such results as:

% diff -s /COPYRIGHT /var/run/rpcbind.sock
diff: /var/run/rpcbind.sock: Operation not supported
Files /COPYRIGHT and /var/run/rpcbind.sock are identical

% diff -s /COPYRIGHT /etc/master.passwd
diff: /etc/master.passwd: Permission denied
Files /COPYRIGHT and /etc/master.passwd are identical

Create a D_ERROR status which is returned in such cases, and
print_status() then deals with that status seperately from D_SAME

PR: 252614
MFC after: 1 week

(cherry picked from commit fefb3c46a80fdde6f307e73a2b5b5aed806df1ce)