Several minor fixes and harmonisation items for Makefiles:

* Use the same mechanism for verbose/non-verbose output in two files
  ("$(Q)"), for all commands.
* Use calls to "QUIET_INSTALL" and equivalent in Makefile. In
  particular, use "call(descend, ...)" instead of "make -C" to run
  documentation targets.
* Add a "doc-clean" target, aligned on "doc" and "doc-install".
* Make "install" target in Makefile depend on "bpftool".
* Remove condition on DESTDIR to initialise prefix in doc Makefile.
* Remove modification of VPATH based on OUTPUT, it is unused.
* Formatting: harmonise spaces around equal signs.
* Make install path for man pages /usr/local/man instead of
  /usr/local/share/man (respects the Makefile conventions, and the
  latter is usually a symbolic link to the former anyway).
* Do not erase prefix if set by user in bpftool Makefile.
* Fix install target for bpftool: append DESTDIR to install path.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>