completion/system: give up hope

Give up and accept defeat that bash-completion can't reasonably be audited for unbound parameters. Wrap invocation with disabling strictness, and restore after if it was enabled.

completion/available/system.completion.bash

@@ -1,8 +1,16 @@
 #!/usr/bin/env bash
-
+#
 # Loads the system's Bash completion modules.
 # If Homebrew is installed (OS X), it's Bash completion modules are loaded.
 
+if shopt -qo nounset
+then # Bash-completion is too large and complex to expect to handle unbound variables throughout the whole codebase.
+	BASH_IT_RESTORE_NOUNSET=true
+	shopt -uo nounset
+else
+	BASH_IT_RESTORE_NOUNSET=false
+fi
+
 if [[ -r /etc/bash_completion ]] ; then
   # shellcheck disable=SC1091
   source /etc/bash_completion
@@ -23,3 +31,9 @@ if [[ "$(uname -s)" == 'Darwin' ]] && _command_exists brew ; then
     source "$BREW_PREFIX"/etc/profile.d/bash_completion.sh
   fi
 fi
+
+if $BASH_IT_RESTORE_NOUNSET
+then
+	shopt -so nounset
+fi
+unset BASH_IT_RESTORE_NOUNSET