msmtp: add new option to delegate use to msmtpq

[octvs]

Add option programs.msmtp.offlineSupport which when enabled delegates the usage of msmtp to msmtpq script. This is achieved by an internal option msmtpCommand which should be called when msmtp is used by another module.

Resolves #2933.

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[octvs]

There are two concerns that I see on this. First is to generalize the msmtpCommand variable introduced here to be a account-wide accounts.email.accounts.<name>.sendMailCommand as in the modules of alot and astroid, where the newly introduced account-wide variable can fill as the default of submodules (i.e. alot and astroid).

Another is to whether to include necessary flags, --read-envelope-from --read-recipients, to be part of the command. msmtpq script by itself doesn't work as a command without these flags, which is supported by the fact that everywhere that it is used, it is accompanied by them.

Would be great to hear other opinions, so that we reduce the chance to break other workflows.