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* legacy: output --preprocess result on ExecStdout

* legacy: removed redundant argument filters

* legacy: moving path-relativization code out of PrepareCommand

* legacy: removed i18n on unneded message

* legacy: replacing ParseCommandLine with the equivalent library call

* legacy: removed parameter that happens to be always false

* legacy: removed constants.MSG_PATTERN_MISSING from i18n

* Let ExecRecipe return the command executed

This prepares for building a compilation database later. The returned
command is not currently used anywhere yet, so this commit should not
change behaviour.

* Fixed typo
Co-authored-by: Matthijs Kooijman <matthijs@stdin.nl>