Changes to std_labels.h

1. On line 36 (of Release 2.1.1), strings.h was changed to string.h for ANSI compatibility. This change was made in a portion of std_labels.h that vendors are encouraged to change to adapt to their own libraries.

2. Added declarations of l_t3_compartments, l_t3_markings, l_t4_compartments, l_t4_markings, l_t5_compartments, and l_t5_markings.

3. The l_w_type flags SPECIAL_INVERSE and SPECIAL_COMPARTMENTS_INVERSE were defined.