cli: --check-flag

[shlomi-noach]

Running gh-ost with --check-flag is a way of testing for existence of other flags. You may be using different versions of gh-ost, possibly testing a new version that has a new feature. It's now possible to validate that gh-ost supports a flag.

Running with --check-flag doesn't actually do anything. The app terminates immediately, with:

• return code 0 if all flags are good (flags that normally require a value must get such value)
• return code 1 if one or more flags are not recognized, or are missing a value.

Examples:

Returning exit code 0

• gh-ost --check-flag --initially-drop-socket-file
• gh-ost --check-flag --initially-drop-socket-file --nice-ratio 0

Returning nonzero exit code

• gh-ost --check-flag --no-such-flag
• gh-ost --check-flag --initially-drop-socket-file --nice-ratio

[shlomi-noach]

We will be using this new feature in our automation scripts; for example, we might want to execute a resurrect ; the flag will let us know if --resurrect is supported.