feat(completions/system): integrate bash-completion loader in lib/bou…

…rne-shell

The search that has been in lib/bourne-shell.sh is slightly different
from that in completions/system.completion.sh.  We integrate the
former into the latter.

* We add a search location
  `/usr/share/bash-completion/bash_completion` for bash-completion.
  This is the standard location for bash-completion v2.  We have been
  only checking /etc/bash_completion which is bash-completion v1.

* We also add a guard for the POSIX mode.  Older versions of
  bash-completion have an issue with the POSIX mode.  In particular,
  bash-completion v1 can only be used with with the macOS Bash 3.2,
  but bash-completion v1 does not work well in the POSIX mode.

* We also add a guard for already loaded bash-completion.  Other
  system configuration might already load bash-completion.  We skip
  loading bash-completion when we detect an existing bash-completion
  settings in tbe current shell environment.

completions/system.completion.sh

@@ -2,6 +2,14 @@
 
 # Loads one of the system's Bash-completion modules.
 
+# If bash-completion is already enabled (by e.g. /etc/bash.bashrc sourced from
+# /etc/profile or directly executed by Bash), we skip loading bash-completion.
+[[ ${BASH_COMPLETION_VERSINFO-} ]] && return 0
+
+# We skip loading bash-completion in the POSIX mode.  Older versions of
+# bash-completion do not work in the POSIX mode.
+shopt -oq posix && return 0
+
 # If Homebrew is installed (OS X), its Bash completion module is loaded.
 if is_os darwin && _omb_util_command_exists brew; then
   BREW_PREFIX=$(brew --prefix)
@@ -19,7 +27,12 @@ if is_os darwin && _omb_util_command_exists brew; then
   fi
 fi
 
-if [[ -f /etc/bash_completion ]]; then
+if [[ -f /usr/share/bash-completion/bash_completion ]]; then
+  # bash-completion v2 is installed at this location
+  source /usr/share/bash-completion/bash_completion
+  return "$?"
+elif [[ -f /etc/bash_completion ]]; then
+  # bash-completion v1 is installed at this location
   source /etc/bash_completion
   return "$?"
 fi