add &CORE::chdir (properly)

[mauke]

Previously, CORE::chdir was not "ampable", i.e. it could not be called
through a coderef or with a sigil: &CORE::chdir($dir);

The only thing allowed was to make a compile-time alias, whose calls
would then be inlined to chdir again:

BEGIN { *foo = \&CORE::chdir; }
foo ".";  # actually does chdir "."

However, this was broken if the function call used parentheses, in which
case chdir would always try to use its argument as a filehandle, not the
name of a directory:

BEGIN { *foo = \&CORE::chdir; }
foo(".");
# Triggers a warning:
# chdir() on unopened filehandle . at file line 2.

This commit fixes the problem and makes chdir fully "ampable", so
&CORE::chdir() no longer triggers the &CORE::chdir cannot be called directly error.

[tonycoz]

The three different chdir() PRs seem to me like they'd conflict with each other.

[mauke]

The three different chdir() PRs seem to me like they'd conflict with each other.

True, but they're all mine, so I know how to resolve/rebase things. I just wanted the PRs to be independently reviewable at first.