Add bootstrap script — copy the .rc files to the home directory

Use this script the easily create a symbolic link to the configuration
files from our home directory. This first version simply ignores those
files which already exist in ~, without doing anything destructive.
Allow the user to overwrite configuration files

We use the term 'overwrite', but of course what the script does is to
remove the exiting .rc file before attempting to create the symlink in
the home directory. If the user enters an unrecognized command it is
assumed to mean 'ignore' — that is, we do not do anything by default.
Add option to ignore all existing .rc files — [S]kip all
Add option to replace all existing .rc files — [O]verwite all
Ignore leading and trailing whitespaces in the user's response
Add GNU Screen configuration file
Redefine command key to C-z
Enable dynamic windows titles

The shelltitle command makes it possible that while we are running a
command the title of the window automatically changes to that command.
Note that this involves modifying .bashrc so that our shell outputs a
null title-escape-sequence as part of the prompt, but that file is not
yet ready to be version controlled and shown to the world.
Use Rafael Magana's hardstatus line

The main difference with respect to our previous hardstatus line is
that this one includes the current time and date. Also, the list of
windows is padded to the display's width — i.e., they are centered.
I have modified the command so that red background is used to mark
the active window instead of parentheses.

Kudos to Rafael Magana for making his screenrc available on GitHub:
https://github.com/rafmagana/screenrc
Revert "Redefine command key to C-z"

The idea of rebinding the command key of screen to C-z, as we did in
commit 4e968a6fbc7a9a24b49441, was not as good as it looked at first.
An unexpected consequence of getting used to this key binding is that
I keep suspending the current foreground job when working on machines
where my screenrc is not installed, which is really annoying. I must
resign myself to use `C-a a` to send the command character (C-a) to
Emacs — this is indeed mentioned in the man page for screen!