muscles: be sure that %code snippets are not glue together on a single line

Recently "braceless" in the parser was changed so that an eol was no
longer added to the value.  This is not correct when a %code is used
multiple times, because the syncline of the next snippet might be
appended to the last (and not ended) line of the previous snippet.

* src/muscle-tab.h (muscle_grow): Make it private.
* src/muscle-tab.c (muscle_grow): Accept a fourth argument: a required
terminator.
Adjust callers.
* tests/input.at (Multiple %code): New.