libxbps::xbps_humanize_number(): use common values and hide implementation details from API.

So now its prototype is the following:

int xbps_humanize_number(char *buf, int64_t bytes)

It is a wrapper around NetBSD's humanize_number(3) which uses 6 digits for max
length, HN_AUTOSCALE and HN_NOSPACE|HN_DECIMAL. All users have been updated.

include/xbps_api.h

@@ -217,15 +217,7 @@ const char *xbps_fetch_error_string(void);
 
 /*@}*/
 
-/* From lib/humanize_number.c */
-#define HN_DECIMAL		0x01
-#define HN_NOSPACE		0x02
-#define HN_B			0x04
-#define HN_DIVISOR_1000		0x08
-#define HN_GETSCALE		0x10
-#define HN_AUTOSCALE		0x20
-
-int xbps_humanize_number(char *, size_t, int64_t, const char *, int, int);
+int xbps_humanize_number(char *, int64_t);
 
 /**
  * @ingroup dircreate