[scaffold] *_oml.h inconditionally declare helpers

This caused problems in multi-file projects as registration and
injection helpers were only defined for the main file (the one defining
OML_FROM_MAIN, that is). Other files therefore wouldn't be able to
access these.

Bug found by Guillaume Jourjon.

Signed-off-by: Olivier Mehani <olivier.mehani@nicta.com.au>
Cleanup and doxument public API code and headers

refs: #372

Signed-off-by: Olivier Mehani <olivier.mehani@nicta.com.au>
Miscellaneous small fixes

* Don't reset a filter's sample size when processing an MS as that
  filter already does it when generating an output;
* Const-correctness for oml_value_set;
* Better memory management on error in public API.

Signed-off-by: Olivier Mehani <olivier.mehani@nicta.com.au>
Cleanup SQLite3 journals generated by tests

Signed-off-by: Olivier Mehani <olivier.mehani@nicta.com.au>

Conflicts:
	test/server/Makefile.am