udev-rules: drop truncated import output line

udev-rules: drop truncated import output line

When IMPORT{program} output was truncated, the code intended to drop
the last incomplete result line before importing properties.

It edited the command buffer instead of the output buffer, so the
incomplete line could still be parsed and imported.

Trim the result buffer and cover the behavior with the existing IMPORT
test.

Follow-up for 6b6e471a325bf149839c5c822b4ae3e66cb1d9a3.

Reproducer:

Configure an IMPORT{program} rule whose output exceeds UDEV_LINE_SIZE
and ends with an incomplete property line, for example:

TRUNCATED_OK=yes
TRUNCATED_BAD=<very long value without terminating newline>

Only TRUNCATED_OK should be imported. Before this fix, the truncation
handler edited the command buffer instead of the output buffer, so
TRUNCATED_BAD could still be parsed from the truncated result.