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>

include/libi3.h

@@ -134,6 +134,20 @@ char *sstrdup(const char *str);
  */
 int sasprintf(char **strp, const char *fmt, ...);
 
+/**
+ * Wrapper around correct write which returns -1 (meaning that
+ * write failed) or count (meaning that all bytes were written)
+ *
+ */
+ssize_t writeall(int fd, const void *buf, size_t count);
+
+/**
+ * Safe-wrapper around writeall which exits if it returns -1 (meaning that
+ * write failed)
+ *
+ */
+ssize_t swrite(int fd, const void *buf, size_t count);
+
 /**
  * Build an i3String from an UTF-8 encoded string.
  * Returns the newly-allocated i3String.