Treat a trailing backslash as an error

Commit a2a5291b changed the parser to reject unfinished quoted
strings. Unfortunately it introduced an error where a trailing
backslash would case an infinite loop. Of course this must fixed, but
the question is what to to instead. Allowing trailing backslashes and
treating them as normal characters would be one option, but this seems
suboptimal. First, there would be inconsistency between handling of
quoting and of backslashes. Second, a trailing backslash is most
likely an error, at it seems better to point it out to the user than
to try to continue.

Updated rules:
ExecStart=/bin/echo \\ → OK, prints a backslash
ExecStart=/bin/echo \ → error
ExecStart=/bin/echo "x → error
ExecStart=/bin/echo "x"y → error

(cherry picked from commit ba774317)

Conflicts:
	src/test/test-strv.c