errno should be queried immediately after a libc function call fails,
calling it later on in QSystemError::stdString() may be too late.
Part of the goal of this, and similar, changes is removing the default
value of the stdString() method's parameter, to signify to users of that
method that errno should be stored ASAP if there is an error.
If there is any intervening code that may call a system/libc function
store errno in a local int var, otherwise use it directly in the
QSystemError::stdString() call, which takes by value.
Task-number: QTBUG-115199
Change-Id: If3f601a023ed0014e260089771220668dad88be8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
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