dpkg-gensymbols: override the check level with an environment variable

dpkg-gensymbols: override the check level with an environment variable

* scripts/dpkg-gensymbols.pl: The environment variable
DPKG_GENSYMBOLS_CHECK_LEVEL can override the level of checks made by
dpkg-gensymbols. This offers the possibility to unofficial ports to ignore
errors due to symbols files that do not correspond.