Cleaning error logs for rules applying utilities

Removed perror logs from main utilities files. Moved error logs
inside /utils/common.c. Errors are logged from I/O operations and
applying rules/cipso from files.
Add smack_label_length() function

Kernel does not validate the Smack label - instead the label will be
cut on the first incorrect character (after parsing at least one
correct character).

This function gives the user possibility to verify the correctness
of the Smack label before use and calculate labels length.

Additionally, all string length calculations are replaces with this
function to make implementation safer.

[jarkko.sakkinen@linux.intel.com:
 did some modifications:
 - smack_is_label_valid() -> smack_label_length()
 - libsmack.c:
   * return length
   * loop invariant had off-by-one error
   * cosmetic: "++i" not "i++"
 - libsmack.h:
   * updated documentation
   * cosmetic: formatting errors in  @param and @return]
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().
Fix man pages

Regression from 1edba54.
Regression fix: don't modify smack_accesses while applying the rules.

Regression introduced in eaf908fd caused access_type field to be modified,
when smack_accesses_apply() is called with clear set to true.

This patch reverts that, restoring invariant that smack_accessess_apply()
doesn't modify given rule set.