Simplified `bash-completion-add-to-alist'; small docstrings fixes.
Moved code to initialize `bash-completion-alist' for a process to its own function: `bash-completion-initialize'.
All read-access to `bash-completion-alist' through the new function `bash-completion-specification'.
Move call to `bash-completion-initialize' from `bash-completion-send' to `bash-completion-specification'.
This fixes a bug since `bash-completion-alist' must be already
initialized when `bash-completion-generate-line' is called to find the
completion spec for a given command.