Force loading of m-buffer on opening.

Some of the macros in m-buffer have indent forms, so this
file needs to loaded as soon as we open linked-buffer files
to ensure accurate indentation.
Treat start of line properly in blocks.

Previously, deleting the uncommented buffer to the beginning of the line
would leave the comments around. We now expand any change at the
beginning of the line in the uncommented buffer to the beginning of the
line in the commented buffer (i.e. affecting comments).