Move {safe_,}rename() from install{man,perl} into install_lib.pl

installman's rename() was identical to installperl's safe_rename() in all
but name (and whitespace), so de-duplicate by moving the code to
install_lib.pl
Move the wrapper for File::Path::mkpath() to install_lib.pl

installperl and installman call File::Path::mkpath with identical arguments
and options, so move the repeated code into a single place.
In install_lib.pl, no need to Config->import for the relocatableinc setup.

require Config; within the BEGIN block instead of using it outside it to
save creating one implicit BEGIN block, and running its import twice.

Remove the require 5.004; as the require of Config will fail if running
with anything other than the version about to be installed.

Note in installperl and installman that install_lib.pl imports Config.
install_lib.pl's samepath() should not warn if $p1 does not exist.

If $p1 is a non-existent path, then the two paths can't be the same, so
samepath() should promptly return false.
Merge the install_lib.pl/installman/installperl refactoring into blead.