Fix option parsing so it works across possible SH implementations

[alerque]

Per the yucky discovery here our current option parsing is not robust across SH implementations. Whatever @RichiH's has works. My systems don't.

On my systems /bin/sh is a symlink to bash, which should be triggering its internal POSIX compatibility mode. Whatever you think of this it's a pretty common arrangement.

On the other hand I tested dash and I get different results:

$ ./vcsh -v foreach -g echo foo
verbose mode on
vcsh 1.20141026
vcsh: verbose: foreach begin
que-atom:
git: 'echo' is not a git command. See 'git --help'.

The most similar command is
        fetch

$ dash ./vcsh -v foreach -g echo foo
verbose mode on
vcsh 1.20141026
vcsh: verbose: foreach begin
que-atom:
foo

[RichiH]

I seem to remember very, very, very dimly that Bash's POSIX compatibility mode not actually being POSIX compatible is what prompted me to hand-craft the option parsing.

@mirabilos might be able to help with his POSIX-fu.

[alerque]

It's possible that is the case, but the hand crafted version isn't working either. Different error (it fails early) but it still doesn't work. The safe-harbor release under Bash's pseudo POSIX gives:

$ vcsh -v foreach -g echo foo
verbose mode on
vcsh 1.20141026
/usr/bin/vcsh: illegal option -- g
...