Use initalization instead of explicit zeroing

Before, we would initialize many fields twice: first
by filling the structure with zeros, and then a second
time with the real values. We can let the compiler do
the job for us, avoiding one copy.

A downside of this patch is that text gets slightly
bigger. This is because all zero() calls are effectively
inlined:

$ size build/.libs/systemd
         text    data     bss     dec     hex filename
before 897737  107300    2560 1007597   f5fed build/.libs/systemd
after  897873  107300    2560 1007733   f6075 build/.libs/systemd

… actually less than 1‰.

A few asserts that the parameter is not null had to be removed. I
don't think this changes much, because first, it is quite unlikely
for the assert to fail, and second, an immediate SEGV is almost as
good as an assert.

src/vconsole/vconsole-setup.c

@@ -179,20 +179,18 @@ static int font_load(const char *vc, const char *font, const char *map, const ch
  * to apply a new font to all VTs.
  */
 static void font_copy_to_all_vcs(int fd) {
-        struct vt_stat vcs;
-        int i;
-        int r;
+        struct vt_stat vcs = {};
+        int i, r;
 
         /* get active, and 16 bit mask of used VT numbers */
-        zero(vcs);
         r = ioctl(fd, VT_GETSTATE, &vcs);
         if (r < 0)
                 return;
 
         for (i = 1; i <= 15; i++) {
                 char vcname[16];
-                struct console_font_op cfo;
                 int _cleanup_close_ vcfd = -1;
+                struct console_font_op cfo = {};
 
                 if (i == vcs.v_active)
                         continue;
@@ -208,7 +206,6 @@ static void font_copy_to_all_vcs(int fd) {
                         continue;
 
                 /* copy font from active VT, where the font was uploaded to */
-                zero(cfo);
                 cfo.op = KD_FONT_OP_COPY;
                 cfo.height = vcs.v_active-1; /* tty1 == index 0 */
                 ioctl(vcfd, KDFONTOP, &cfo);