fix dcs handling, and add a test for it

[doy]

fixes https://github.com/jwilm/vte/issues/28.

it turns out that there were a couple different issues with dcs parsing, and this should resolve all of them:

• the final character of the dcs introducer was not being passed to the hook method, making it impossible to determine which dcs sequence was being processed (which is necessary to correctly handle the put method)
• the last parameter was being incorrectly dropped

the test tests the dcs sequence DECUDK documented in the xterm documentation.

additionally, you can see the difference in the parselog output before this change:

$ perl -E'print "foo\eP0;1|17/ab\x9cbar"' | cargo run -q --example parselog
[print] 'f'
[print] 'o'
[print] 'o'
[hook] params=[0], intermediates=[], ignore=false
[put] 31
[put] 37
[put] 2f
[put] 61
[put] 62
[unhook]
[print] 'b'
[print] 'a'
[print] 'r'

compared to after:

$ perl -E'print "foo\eP0;1|17/ab\x9cbar"' | cargo run -q --example parselog
[print] 'f'
[print] 'o'
[print] 'o'
[hook] params=[0, 1], intermediates=[], ignore=false, char='|'
[put] 31
[put] 37
[put] 2f
[put] 61
[put] 62
[unhook]
[print] 'b'
[print] 'a'
[print] 'r'