There are some enum typedefs with the enum name:
typedef enum _name_t { ... } name_t;
We don't need or use the enum names _name_t for anything, and not all
of the enum typedefs have them. We have the typedefs specifically to
use the typedef name.
Use the anonymous enum in the typedefs:
typedef enum { ... } name_t;
libnotmuch_util.a is supposed to be usable from the library and the
CLI, but much the library is compiled as C++. Add in appropriate
wrapping to prevent symbol mangling. These wrappers already existed in
string-util.h; it seems better to be consistent.
