Rework shstr to handle bytes and strings; add squote and bquote

Rework shstr to handle bytes and strings; add squote and bquote

These could be smarter, i.e '1 could become "'"1 rather than ''"'"'1',
but we can always improve it later.  And add at last some tests.
Don't rely on compat.quote for strings so that we know we'll have the
same behavior for bytes and strings.

Thanks to Johannes Berg for pointing out an incorrect variable name in
a previous revision.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>