wall: do not use a temporary file.

wall: do not use a temporary file.

The issue with using a temporary file in wall is that wall runs as setgid.
This means that an unprivileged user who runs wall can modify wall's
temporary files, even if those are mode 0600, so the unprivileged user can
edit and effectively suppress the banner. The fix is to simply not use
temporary files.

[kzak@redhat.com: - rewrite growing string functions
                  - use struct buffer
                  - add buf_print() function]

Signed-off-by: Karel Zak <kzak@redhat.com>
Co-Author: Jann Horn <jann@thejh.net>