#582 file command enhanced to use effective user-ids

[Larry W. Virden]

Currently the file command performs all its functions using the real user-id of the user. However, there are times when a tcl script needs to run with an effective user-id different than one's real user-id.

When this happens, one can always perform an "open filename r" with a catch to see if the file will open, but it would be useful if one could just use file readable to test the file.

There are likely other scenarios where other of the file command's subcommands would be useful to use in a "set-userid" scenario.