flag: support func flags without arguments

[fnxpt]

Current implementation forces a func flag to have a value
When displaying the usage is possible to see a "value" required for the flag and when you execute it without flag you receive the error flag needs an argument

Converted funcValue to a struct and create 2 additional functions to avoid breaking changes

#53747

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Message from Ian Lance Taylor:

Patch Set 1: Hold+1

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 3: Hold+1

(4 comments)

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Message from Ian Lance Taylor:

Patch Set 3: -Hold

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Message from Rob Pike:

Patch Set 3:

(1 comment)

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Message from Carl Johnson:

Patch Set 3:

(1 comment)

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Message from Ian Lance Taylor:

Patch Set 3:

(2 comments)

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[fnxpt]

@carlmjohnson, sorry for not replying I've been busy with other stuff and I kind of forgot about this... thanks for fixing it