util: rename format_timestamp_pretty() to format_timestamp_relative() because that describes much better what it does

src/shared/time-util.c

@@ -142,7 +142,7 @@ char *format_timestamp(char *buf, size_t l, usec_t t) {
         return buf;
 }
 
-char *format_timestamp_pretty(char *buf, size_t l, usec_t t) {
+char *format_timestamp_relative(char *buf, size_t l, usec_t t) {
         usec_t n, d;
 
         n = now(CLOCK_REALTIME);

src/shared/time-util.h

@@ -71,7 +71,7 @@ usec_t timeval_load(const struct timeval *tv);
 struct timeval *timeval_store(struct timeval *tv, usec_t u);
 
 char *format_timestamp(char *buf, size_t l, usec_t t);
-char *format_timestamp_pretty(char *buf, size_t l, usec_t t);
+char *format_timestamp_relative(char *buf, size_t l, usec_t t);
 char *format_timespan(char *buf, size_t l, usec_t t);
 
 void dual_timestamp_serialize(FILE *f, const char *name, dual_timestamp *t);