VPP-346 More VPP doc fixes

VPP-346 More VPP doc fixes

- Fix issue in Doxy dependency check when nothing needs to be installed.
  'set -e' and plain '[]' logic don't mix well.
- Fix Makefile snafu when building Doxy output for a single file.
- Include only one of vnet/vnet/buffer.c/dpdk_buffer.c in docs depending on
  DPDKness. This could do with some improvement in future, eg to properly
  align the pre-doxy steps with what Doxy does.
- Fix rendering of 'inline' tag in Doxygen by having it interpret
  always_inline as "inline static".
- Bunch of duplicate CLI command structure names that confused docs and may
  one day have caused debugging issues.
- Several other Doxygen syntax issues fixed, like documenting non-existant
  parameters (usually just the wrong parameter name, typos, etc)

Change-Id: Ia8cca545e5de9f8750602bffa3c4548acc8971aa
Signed-off-by: Chris Luke <chrisy@flirble.org>