RFE: support "--" flag in `su` to circumvent additional shell

[vcaputo]

There's a convention in the GNU userspace for a bare '--' arg to indicate "everything in argv following the -- is not to be interpreted as a flag, just use as input to the program"

In the case of su, when you want su to run a command, you currently must use "-c/--command WORD_REPRESENTING_COMMAND", which will then be interpolated by another shell's -c invocation running as the target user.

This can be rather annoying and error prone, depending on the complexity of whatever is being executed via su.

I'd like to propose supporting "--" as an alternative to -c/--command which instead treats whatever remains in argv as the argv to supply directly to execve(), bypassing any further shell invocation or subsequent word splitting.

[hallyn]

I'm all for this. Will try to do it if I find some spare time.

[vcaputo]

After glancing at the code and actually reading the man page I see there's already existing semantics for '--' to pass subsequent argv entries on as flags.

So it looks like this should instead add a new flag to suppress using any interpretive shell when running the command, and suppress argv manipulation like adding the '-c' shell command flag.

How about supporting "--exec-command" / "-e" to suppress any "$shell -c", directly calling execve() on the command, with any arguments after '--' supplied as the rest of argv?

[vcaputo]

I took a stab at implementing this #254
PTAL