add Dbxref and Ontology_term syntax checking
src_clean
fix case
allow compiling with assert=no
allow compiling with amalgamation=yes
adjust XRF checker behaviour to be similar to type checker
initialise variable differently

This fixes a scan-build warning.
Merge pull request #167 from satta/xrf

Implement XRF_abbr parsing and Dbxref/Ontology_term checking