fix error checking in dev.set
fixed arg error checking in gevents.c
formatting tweak
documented R_DEBUGGER and R_DEBUGGER_ARGS
Merge branch '19-doc' into 20
Merge branch '19-fixes' into 20
Merge branch '19-fixes-from-R-core' into 20
Merge branch '20' into 20-new-news
updated NEWS
Merge branch '20' into 21
Merge branch '20-new-news' into 21