ubxtool: Log the size of the 'svseen' set in "currently receiving" message

[edmonds]

This commit updates the "... currently receiving ..." log message to include the size of the svseen set. It's hard to eyeball this figure with the current log format, especially if the output has been wrapped or truncated.

Example of new output:

Sun, 20 Sep 2020 19:50:25 +0000 swVersion: ROM CORE 3.01 (107888)
Sun, 20 Sep 2020 19:50:25 +0000 hwVersion: 00080000
Sun, 20 Sep 2020 19:50:25 +0000 Extended info: FWVER=SPG 3.01
Sun, 20 Sep 2020 19:50:25 +0000 Extended info: PROTVER=18.00
Sun, 20 Sep 2020 19:50:25 +0000 Extended info: GPS;GLO;GAL;BDS
Sun, 20 Sep 2020 19:50:25 +0000 Extended info: SBAS;IMES;QZSS
Sun, 20 Sep 2020 19:50:25 +0000 Serial number f3044f0761
Sun, 20 Sep 2020 19:50:25 +0000 Entering main loop
Sun, 20 Sep 2020 19:50:29 +0000 src 71 (fix: 3) currently receiving (1): 2,9@1
Sun, 20 Sep 2020 19:51:00 +0000 src 71 (fix: 3) currently receiving (15): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,36@1 6,3@0 6,4@0 6,17@0 6,18@0
Sun, 20 Sep 2020 19:51:31 +0000 src 71 (fix: 3) currently receiving (15): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,36@1 6,3@0 6,4@0 6,17@0 6,18@0
Sun, 20 Sep 2020 19:52:02 +0000 src 71 (fix: 3) currently receiving (15): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,36@1 6,3@0 6,4@0 6,17@0 6,18@0
Sun, 20 Sep 2020 19:52:33 +0000 src 71 (fix: 3) currently receiving (15): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,36@1 6,3@0 6,4@0 6,17@0 6,18@0
Sun, 20 Sep 2020 19:53:04 +0000 src 71 (fix: 3) currently receiving (16): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,11@1 2,36@1 6,2@0 6,3@0 6,4@0 6,18@0
Sun, 20 Sep 2020 19:53:35 +0000 src 71 (fix: 3) currently receiving (16): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,11@1 2,36@1 6,3@0 6,4@0 6,17@0 6,18@0
Sun, 20 Sep 2020 19:54:06 +0000 src 71 (fix: 3) currently receiving (17): 0,10@0 0,15@0 0,18@0 0,20@0 0,23@0 0,27@0 0,32@0 2,4@1 2,5@1 2,9@1 2,11@1 2,36@1 6,3@0 6,4@0 6,12@0 6,17@0 6,18@0