Fix org#2567 device capabilities overwritten

Fix org#2567 device capabilities overwritten

- The problem is that the ?numerous? "dev->capabilities |= CAP_LSEEK"
  are overwritten by dev->device_generic_init() in init_dev() that
  does a "dev->capabilities = device->cap_bits;"

- Now Bacula display the second line below just after the 1st one when
  appending a already used volume
= 01-Dec 09:34 127.0.0.1-sd JobId 1: Volume "Vol0001" previously written, moving to end of data.
+ 01-Dec 09:34 127.0.0.1-sd JobId 1: Ready to append to end of Volume "Vol0001" size=21,857,409
  this can be seen in restart-job-test