MINOR: flags: implement a macro used to dump enums inside masks

MINOR: flags: implement a macro used to dump enums inside masks

Some of our flags have enums inside a mask. The new macro __APPEND_ENUM
is able to deal with that by comparing the flag's value against an exact
one under the mask. One needs to take care of eliminating the zero value
though, otherwise delimiters will not always be properly placed (e.g. if
some flags were dumped before and what remains is exactly zero). The
bits of the mask are cleared only upon exact matches.