[PATCH] mutt-like attachment level

Attached patch adds a mutt-like attachment level. It is called from page
level with PageViewAttach ('V').

The name of the attachment is displayed at the left side. If no name is
available, '<no name>' is used. At the right side you will find
content-type, encoding, charset and line-count.

Each attachment can be viewed (AttachSelect ('^J', '<CR>')), saved to
disk (AttachSave ('s')) and piped to command (GlobalPipe ('|')).

Pipe uses the raw attachment, view and save the decoded one.

ShellEscape, Help and ToggleHelpDisplay are also implemented.

Known bug: If the name has a star ('*') at the second position, that
star becomes inverse. The reason is draw_arrow_mark() from screen.c,
which inverses the line at MARK_OFFSET when it finds a star there.

Dennis