all: Refactor cmd/syncthing creating lib/syncthing (ref #4085) (#5805)

* add skeleton for lib/syncthing

* copy syncthingMain to lib/syncthing (verbatim)

* Remove code to deduplicate copies of syncthingMain

* fix simple build errors

* move stuff from main to syncthing with minimal mod

* merge runtime options

* actually use syncthing.App

* pass io.writer to lib/syncthing for auditing

* get rid of env stuff in lib/syncthing

* add .Error() and comments

* review: Remove fs interactions from lib

* and go 1.13 happened

* utility functions
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