misc: some coverity fixes

In rough order of seriousness:

1. Fix clock_gettime() in pmtud so that it's always called, as variable 'clock_now' is always read.
2. Allow space for trailing NUL in libnozzle device names
3. Fix api_nozzle_run_updown_test so it can run out of the build tree
4. Disallow a 0 length prefix in libnozzle
5. Fix potential use of NULL pointer on doxyxml
6. Free 'name' in doxyxml as it's *not* in the map any more
7. Fix dead code in libknet API functions left by code changes