Add --exclude-from (-X), and ignore specified package names.

--exclude-from (-X) reads a list of exclude specs from a file.

Exclude specs can optionally specify a package name:
  <packagename>:<messageid>
  <packagename>
  <messageid>
The package name must be exact (wildcards not supported, and
must not include version/arch/build/tag/.t?z).

Essentially, this implements a 'whitelist file'.

Signed-off-by: David Spencer <baildon.research@googlemail.com>
man_check: Tune to reduce false positives.

Signed-off-by: David Spencer <baildon.research@googlemail.com>