backtrace: Try to find absolute executable path

This allows us to get backtraces from test_runner which does not start
iwd from a current working directory that is a parent of the iwd
executable.

src/backtrace.c

@@ -156,14 +156,33 @@ static void signal_handler(int signo)
 	exit(EXIT_FAILURE);
 }
 
-void __iwd_backtrace_init(const char *program)
+void __iwd_backtrace_init()
 {
 	static char path[PATH_MAX];
+	static char cwd[PATH_MAX];
 	struct sigaction sa;
 	sigset_t mask;
+	ssize_t len;
 
-	program_exec = program;
+	/* Attempt to get the full path to our executable */
-	program_path = getcwd(path, sizeof(path));
+	len = readlink("/proc/self/exe", path, sizeof(path) - 1);
+	if (len > 0) {
+		int i;
+		path[len] = '\0';
+
+		for (i = len - 1; i >= 0; i--) {
+			if (path[i] != '/')
+				continue;
+
+			program_exec = path;
+			break;
+		}
+	}
+
+	if (program_exec == NULL)
+		return;
+
+	program_path = getcwd(cwd, sizeof(cwd));
 
 	sigemptyset(&mask);
 	sa.sa_handler = signal_handler;

src/backtrace.h

@@ -20,6 +20,6 @@
  *
  */
 #ifdef __GLIBC__
-void __iwd_backtrace_init(const char *program);
+void __iwd_backtrace_init();
 void __iwd_backtrace_print(unsigned int offset);
 #endif

src/main.c

@@ -197,7 +197,7 @@ int main(int argc, char *argv[])
 	l_debug_enable("*");
 
 #ifdef __GLIBC__
-	__iwd_backtrace_init(argv[0]);
+	__iwd_backtrace_init();
 #endif
 
 	l_info("Wireless daemon version %s", VERSION);