bug#31743: 27.0.50; [feature request] add command to ERC to erase scrollback text

[Jonathan Kyle Mitchell]

A recent discussion[1] in help-gnu-emacs revealed that a user
expected ERC's /CLEAR command (i.e. erc-cmd-CLEAR) to erase the buffer
contents instead of just recentering the display.

[1] http://lists.gnu.org/archive/html/help-gnu-emacs/2018-06/msg00041.html

I see two options to handle this:
 1) Modify the existing erc-cmd-CLEAR command to also erase the buffer
 contents.
 2) Add a new command to erase the buffer contents and keep the existing
 CLEAR command that just clears the window by recentering the text to be
 above the window, preserving the scrollback text.

In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30)
 of 2018-06-06 built on fedorabox
Repository revision: 4a51deb993d923767f0eddd4f350e636fe8d7c0b
Windowing system distributor 'Fedora Project', version 11.0.11906000
System Description: Fedora 28 (Twenty Eight)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --with-modules --with-xwidgets'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES THREADS XWIDGETS LIBSYSTEMD JSON
LCMS2

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date elec-pair
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lisp-float-type mwheel term/x-win x-win term/common-win x-dnd tool-bar
dnd fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
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custom widget hashtable-print-readable backquote dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting xwidget-internal
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 94457 8735)
 (symbols 48 20180 1)
 (miscs 40 37 145)
 (strings 32 28224 2094)
 (string-bytes 1 741382)
 (vectors 16 14073)
 (vector-slots 8 493380 15320)
 (floats 8 48 69)
 (intervals 56 206 0)
 (buffers 992 11))

--
Jonathan Kyle Mitchell