sh: fix spurious newline slips in backquote expansion

sh: fix spurious newline slips in backquote expansion

From FreeBSD revision 1.43:
    Instead of eating trailing newlines after inserting them into the
    output buffer, don't insert them at all.  This prevents a buffer
    *underrun* when the substitution consists completely of newlines
    (e.g. `echo`) and the byte before the source buffer to which p
    points is a '\n', in which case more characters would be removed
    from the output buffer than were inserted.

DragonFly-bug: <http://bugs.dragonflybsd.org/issue1588>