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.

src/dlhook.h

@@ -8,11 +8,11 @@ extern "C" {
 
 // nm_dlhook takes a lib handle from dlopen and redirects the specified symbol
 // to another, returning a pointer to the original one. Only calls from within
-// that library itself are affected (because it replaces that library's GOT). If
-// an error occurs, NULL is returned and if err is a valid pointer, it is set to
-// a malloc'd string describing it. This function requires glibc and Linux. It
-// should work on any architecture, and it should be resilient to most errors.
-NM_PRIVATE void *nm_dlhook(void *handle, const char *symname, void *target, char **err_out);
+// that library itself are affected (because it replaces that library's GOT).
+// This function requires glibc and Linux. It should work on any architecture,
+// and it should be resilient to most errors. If it fails, NULL is returned and
+// nm_err is set.
+NM_PRIVATE void *nm_dlhook(void *handle, const char *symname, void *target);
 
 #ifdef __cplusplus
 }