Add an option to save the log to a file

• Factor out cli_log_func

  Moving this log function to "utils" allows us to avoid a lot of duplication.

• wrap: Use the log handler instead of g_printerr

  We have our own log handler, so there is no need to use g_printerr() directly.

• Add an option to save the log to a file

  With the --log-to-file option it is now possible to save the log of pressure-vessel (and the wrapped command) to a file, instead of printing it in "stderr".

  This should ease the required steps that users need to follow in order to report an issue.

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/cc @smcv

The complementary change to steamlinuxruntime is steamlinuxruntime!26 (merged)