Constify linetables

Linetables no longer change after they are created.  This patch
applies const to them.

Note there is one hack to cast away const in mdebugread.c.  This code
allocates a linetable using 'malloc', then later copies it to the
obstack.  While this could be cleaned up, I chose not to do so because
I have no way of testing it.

Approved-By: Simon Marchi <simon.marchi@efficios.com>