More cleanups.
Don't add any commands beginning with a whitespace to the `comint' history.
Renamed `comint-previous-output-boundaries' to `comint-boundaries-of-previous-output'.
Do not truncate shellbuffers automatically anymore.
First attempt at a `shell-man' command with too many rough edges for now.
Turn off automatic validation in `nxml-mode' for the sake of large XML buffers.
Bugfix for 28d7491.
Buffers in `diff-mode' are no more read-only by default.