hwclock: size optimizations

libbb/time.c: new file, introducing monotonic_us()
pscan, traceroute, arping: use it instead of gettimeofday
ping, zcip: TODO

function                                             old     new   delta
monotonic_us                                           -      89     +89
find_pair                                            164     180     +16
.rodata                                           129747  129763     +16
refresh                                             1144    1152      +8
............
timeout                                                8       4      -4
static.start                                           8       4      -4
last                                                   8       4      -4
parse_conf                                          1303    1284     -19
time_main                                           1149    1124     -25
gettimeofday_us                                       39       -     -39
arping_main                                         2042    1969     -73
hwclock_main                                         594     501     -93
catcher                                              485     380    -105
traceroute_main                                     4300    4117    -183
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 8/11 up/down: 157/-562)        Total: -405 bytes

include/libbb.h

@@ -200,6 +200,7 @@ struct sysinfo {
 };
 int sysinfo(struct sysinfo* info);
 
+unsigned long long monotonic_us(void);
 
 extern void chomp(char *s);
 extern void trim(char *s);