refactor: Align group message sending with other send functions.

[iphydf]

None of the others use out parameters. Also no toxcore function uses out parameters for anything other than arrays and errors. This would be a first, for no good reason.

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This change is 

[codecov]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (6c6c0b1) 68.92% compared to head (c9ca400) 68.99%.

Files	Patch %	Lines
toxcore/tox.c	41.66%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2519      +/-   ##
==========================================
+ Coverage   68.92%   68.99%   +0.06%     
==========================================
  Files          89       89              
  Lines       27781    27782       +1     
==========================================
+ Hits        19149    19167      +18     
+ Misses       8632     8615      -17     

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[iphydf]

@JFreegman FYI this breaks the unreleased API, so toxic will need an update.

[nurupo]

@iphydf the tox-bootstrapd sha256 error annotation is reported twice somehow. Maybe the lifting grep was unnecessary? Or perhaps something else is going on.

[iphydf]

No, it's not unnecessary. It's just that it appears twice, and so it is grepped out twice.

[nurupo]

Oh, I see. The grep is necessary. After the docker build fails, Docker quotes the command that led to the failure and its output once again, which makes the error annotation appear twice in the file we grep.