Compute Standard-Version from lintian file

Closes: 903220
Thanks: Andreas Tille for the idea

Default value is computed from the content of a file delivered by
lintian package. (See the change in Source.pl)

Thw warn_unless code check must be changed because _fetch_std calls
compute_value which checks the computed value, which checks for
warnings, inluding warn_unless eval'ed code, so this call is eval'ed
which calls _fetch_std. This ends up in deep recursion. The change un
warn_unless uses the computed value without any safety check. Which
should be okay: the content of lintian file can be trusted.

Test file is changed so the computed value of Standard-Version is
checked against the content of the lintian file.