* src/location.c (min_int, columns): New.
(location_caret): Compute the line width. Based on it, compute how
many columns must be skipped before the quoted location and truncated
after, to fit the sceen width.
* tests/local.at (AT_QUELL_VALGRIND): Transform into...
(AT_SET_ENV_IF, AT_SET_ENV): these.
Define COLUMNS to protect the test suite from the user's environment.