parser_json: Introduce parse_flags_array()

parser_json: Introduce parse_flags_array()

Various objects support a 'flags' property with value usually being an
array of strings. There is a special case, when merely a single flag is
set: The value may be a string representing this flag.

Introduce a function assisting in parsing this polymorphic value. Have
callers pass a parser callback translating a single flag name into a
corresponding value. Luckily, these single flag parsers are very common
already.

As a side-effect, enable the single flag spec for set flags as well and
update the documentation accordingly.

Signed-off-by: Phil Sutter <phil@nwl.cc>