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A default PS1 doesn't make sense: the user will either choose a theme or not
care about a default prompt since they can set it themselves.

Fixes #7054

`gls` seems to be installed by default or on most macOS systems, but its
default color scheme sucks. This fix will make sure to only use it if it
has been customised prior to running OMZ.

Related: #5516, #5520.

Fixes #5520.

Also, force the use of Bourne-style shell syntax with `dircolors -b`.

GNU coreutils ship a color setup command by default which can be used to
set a good default color theme for ls:

  https://www.gnu.org/software/coreutils/manual/html_node/dircolors-invocation.html

Fixes weird autocompletion of directories and named users.
Fix #4864

Allows to use 256 colors in prompt themes.