[Rd] default destfile in download.file()

[Jeroen Ooms]

A nice default value for the `destfile` argument in download.file() would
be `basename(url)` i.e. the name of the downloaded file. This would
correspond to default behavior in many other web/ftp clients and makes code
slightly more concise:

  download.file("https://svn.r-project.org/R/trunk/doc/CRAN_mirrors.csv")
  mirrors <- read.csv("CRAN_mirrors.csv")

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