Refactored error handling (#62)

Instead of using and checking malloc'd error strings formatted with asprintf and passed through a pointer in function arguments, use a global thread-local statically allocated error buffer with functions to set and get it.

This is more efficient, less error-prone, and easier to use than the old method, especially now that NM is larger (meaning that errors are returned through multiple layers) and parts of it are re-used in other things.

.gitignore

@@ -23,6 +23,7 @@ compile_flags.txt
 /src/config.o
 /src/kfmon.o
 /src/action.o
+/src/util.o
 /src/dlhook.o
 /src/generator.o
 /src/action_c.o