Since commit 692c8128 (4 years ago), the std-atomic and mutex-based
based implementations of protected_int*_adjust() have different in a critical
detail:
The std-atomic verisons return the old/original integer value while the
mutex versions (used in most or all .c files) return the new/adjusted integer
value.

Standardize on the std-atomic pattern: if you want the new/adjusted integer
value, use the protected_int*_adjust_fetch() version of these functions.