Fix --mangle-props and --mangle-props=unquoted

Fix --mangle-props and --mangle-props=unquoted

Fixes: #1247

Fix --mangle-props and --name-cache inconsistency.
AST_Dot and AST_Sub properties are now mangled by --mangle-props
without regard to being used in an assignment statement.

Note: if --mangle-props is used then *all* javascript files used must
be uglified with the same mangle options.

Fix the ignore_quoted=true mangle option, also known as
`--mangle-props=unquoted`.  If a given property is quoted anywhere
it will not be mangled in any quoted or non-quoted context.