Changed the default prompt character to ¥ (the yen symbol).

Signed-off-by: Rory Dudley <rory@netc.lu>

Do not run certain commands, except on specific operating systems, when
calling the global run command file. This allows users to avoid having
unaliases and unsets in their user-defined rc file (~/.dwvshrc).

Signed-off-by: Rory Dudley <rory@netc.lu>

In the global /etc/dwvshrc file, alias grep so that it always displays
with color by default.

The `which` command is not necessarily a builtin, however, it can
display shell aliases. This patch adds a line into dist/etc/dwvshrc,
which allows /usr/bin/which to detect our shell aliases, and print them.

Instead of passing a hard-coded value for the prompt, use $PS1. The
default is '|> ', set in dist/etc/dwvshrc.

Use $PATH, instead of a hard-coded PATH from main(). This means that
there is no longer a need to pass around PATH to
repl()/recite()/path::refresh(), since path::refresh() can call env::var
directly.

Since the hard-coded paths were removed, there needs to be some way to
define $PATH. When running the debug build, dwvsh will look in
'dist/etc/dwvshrc' for the initial environment setup. For the release
target, dwvsh will look in '/etc/dwvshrc'. After the global rc file is
sourced, dwvsh will try to source ~/.dwvshrc if it exists, so users can
extend their environment without root access (assuming a release install).