sparse cleanup

Fix various things that sparse complains about:
 - use NULL instead of 0
 - make sure we declare everything properly, or mark it static
 - use proper function declarations ("fn(void)" instead of "fn()")

Sparse is always right.

strbuf.c

@@ -4,7 +4,7 @@
 #include "cache.h"
 
 void strbuf_init(struct strbuf *sb) {
-	sb->buf = 0;
+	sb->buf = NULL;
 	sb->eof = sb->alloc = sb->len = 0;
 }