util: Fix assertion in split() on missing '

      When parsing a unit with a trailing slash after an escaped line break, like

  ExecStart=/bin/echo 'foo \
    bar'

the split() function (through config_parse()) asserted and crashed pid 1:

  Assertion 'current[*l + 1] == quotechars[0]' failed at ../src/shared/util.c:583, function split(). Aborting.

Fix this by returning an error in this case ("trailing garbage").

Add corresponding test case. Also fix the missing "unit" argument of
config_parse_exec() in the comment.

https://launchpad.net/bugs/1447243