Add a safe wrapper for write and fix some warnings

1. Add a function writeall and make swrite wrap that function. Use either writeall or swrite, depending on whether we want to exit on errors or not.
2. Fix warnings when compiling with a higher optimisation level.
(CFLAGS ?= -pipe -O3 -march=native -mtune=native -freorder-blocks-and-partition)

Signed-off-by: hwangcc <hwangcc@csie.nctu.edu.tw>

src/sighandler.c

@@ -70,8 +70,14 @@ static int backtrace(void) {
         int stdin_pipe[2],
             stdout_pipe[2];
 
-        pipe(stdin_pipe);
-        pipe(stdout_pipe);
+        if (pipe(stdin_pipe) == -1) {
+            ELOG("Failed to init stdin_pipe\n");
+            return -1;
+        }
+        if (pipe(stdout_pipe) == -1) {
+            ELOG("Failed to init stdout_pipe\n");
+            return -1;
+        }
 
         /* close standard streams in case i3 is started from a terminal; gdb
          * needs to run without controlling terminal for it to work properly in