Use _Bool for booleans

[zilinc]

Issue by lim060
Fri Sep 4 16:49:07 2015
Originally opened as https://github.csiro.au/ts-filesystems/Cogent/issues/97

---

The _Bool type has better compatibility with C booleans, because truthy values are automatically converted to 1 on assignment. This eliminates bugs like 323f4e4de.

The fix should be as simple as

typedef struct bool_t {
  _Bool boolean;
} bool_t;

c-parser treats _Bools as 8 word so verification should be mostly unaffected.