Fix: smack_label_length(): int -> ssize_t
Remove errno assignments

If we want to report something libsmack specific, then we should
have our own error codes. By using errno values to report libsmack
specific error condition we almost zero their value. By removing
these assignment you can resolved from a changed errno value that
a system call failed.

This patch does not break API contract as use of errno values is
undefined in the API documentation.
Merge remote-tracking branch 'jarkkos/errno-cleanup'
Fix value returned by smack_label_length()

In some cases smack_label_length have returned 0 on incorrect label.
Now smack_label_length() always returns -1 in case of incorrect label.
Use smack_label_length() internally instead strnlen()

Use smack_label_length() to check correctness labels instead strnlen().