editcap: add support for epoch timestamps in `-A` and `-B` options

Inspired in https://gitlab.com/wireshark/wireshark/-/merge_requests/1618.

Tested:

Timestamps on file used for comparison:
```
$ tshark -r test/captures/snakeoil-dtls.pcap -T fields -e frame.time_epoch
1150121069.248818000
1150121069.249193000
1150121069.251152000
1150121069.251384000
1150121069.293686000
1150121069.319315000
1150121075.230753000
1150121105.510885000
1150121105.510934000
```

Before:
```
$ ./build/run/editcap -B 1150121069.3 test/captures/snakeoil-dtls.pcap -
editcap: "1150121069.3" isn't a valid date and time
$ ./build/run/editcap -A 1150121069.3 test/captures/snakeoil-dtls.pcap -
editcap: "1150121069.3" isn't a valid date and time
$ ./build/run/editcap -A 1150121069 test/captures/snakeoil-dtls.pcap -
editcap: "1150121069" isn't a valid date and time
$ ./build/run/editcap -B 1150121069 test/captures/snakeoil-dtls.pcap -
editcap: "1150121069" isn't a valid date and time
```

After:
```
$ ./build/run/editcap -A 1150121069.3 test/captures/snakeoil-dtls.pcap - | tshark -r - -T fields -e frame.time_epoch
1150121069.319315000
1150121075.230753000
1150121105.510885000
1150121105.510934000
$ ./build/run/editcap -A 1150121069 test/captures/snakeoil-dtls.pcap - | tshark -r - -T fields -e frame.time_epoch
1150121069.248818000
1150121069.249193000
1150121069.251152000
1150121069.251384000
1150121069.293686000
1150121069.319315000
1150121075.230753000
1150121105.510885000
1150121105.510934000
$ ./build/run/editcap -B 1150121069.3 test/captures/snakeoil-dtls.pcap - | tshark -r - -T fields -e frame.time_epoch
1150121069.248818000
1150121069.249193000
1150121069.251152000
1150121069.251384000
1150121069.293686000
$ ./build/run/editcap -B 1150121069 test/captures/snakeoil-dtls.pcap - | tshark -r - -T fields -e frame.time_epoch
```

1 files changed, 2 insertions, 1 deletions

diff --git a/editcap.c b/editcap.c
index b5f055caab..175dca693f 100644
--- a/editcap.c
+++ b/editcap.c
@@ -764,6 +764,7 @@ print_usage(FILE *output)
     fprintf(output, "                         given time.\n");
     fprintf(output, "                         Time format for -A/-B options is\n");
     fprintf(output, "                         YYYY-MM-DDThh:mm:ss[.nnnnnnnnn][Z|+-hh:mm]\n");
+    fprintf(output, "                         Unix epoch timestamps are also supported.\n");
     fprintf(output, "\n");
     fprintf(output, "Duplicate packet removal:\n");
     fprintf(output, "  --novlan               remove vlan info from packets before checking for duplicates.\n");
@@ -1284,7 +1285,7 @@ main(int argc, char *argv[])
             nstime_t in_time;
 
             check_startstop = TRUE;
-            if (0 < iso8601_to_nstime(&in_time, optarg)) {
+            if ((0 < iso8601_to_nstime(&in_time, optarg)) || (0 < unix_epoch_to_nstime(&in_time, optarg))) {
                 if (opt == 'A') {
                     nstime_copy(&starttime, &in_time);
                     have_starttime = TRUE;