Rename safer_rename() to rename_if_different(), to accurately describe

Rename safer_rename() to rename_if_different(), to accurately describe
what it does. Use File::Compare rather than Digest::MD5, as the files
are small enough to simply read in. (File::Compare dates from 5.004)
Remove safer_rename_always(), which isn't used.
DRY by replacing the cargo-culted "open or die" with a new function
safer_open(), which uses Gensym (5.002) to create an anonymous file
handle, and opens and binmodes the file, or dies.
This necessitates replacing bareword file handles with lexicals in all
the callers.
Correct the names of files in close or die constructions.

p4raw-id: //depot/perl@33538