Support more Bash internal variables

[larryv]

This PR adds support for the following Bash internal variables (fixing at least #1780 and #2554):

• BASH_ARGV0
• BASH_COMPAT
• BASH_LOADABLES_PATH
• CHILD_MAX
• EPOCHREALTIME
• EPOCHSECONDS
• EXECIGNORE
• INSIDE_EMACS
• PS0
• READLINE_ARGUMENT (planned for bash 5.2)
• READLINE_MARK
• SRANDOM

It also adds READLINE_POINT to the list of variables without spaces (as far as I can tell, it always has an integer value) and removes a redundant instance of COPROC.