ftp: Do not base seq on number of pending commands

Previously, seq was computed as the result of |pending_commands|+1. This
opened the possibility to override queued commands, as well as logging
the same pending ftp reply multiple times.

For example, when commands 1, 2, 3 are pending, command 1 may be dequeued,
but the incoming command then receives seq 3 and overrides the already
pending command 3. The second scenario happens when ftp_reply() selected
command 3 as pending for logging, but is then followed by many ftp_request()
events. This resulted in command 3's response being logged for every
following ftp_request() over and over again.

Avoid both scenarios by tracking the command sequence as an absolute counter.

testing/btest/Baseline/coverage.record-fields/out.default

@@ -162,6 +162,7 @@ connection {
               * ts: time, log=F, optional=F
              }
         * command: string, log=T, optional=T
+        * command_seq: count, log=F, optional=T
         * cwd: string, log=F, optional=T
         * data_channel: record FTP::ExpectedDataChannel, log=T, optional=T
             FTP::ExpectedDataChannel {