When using @cwd %%FOO%%, we must ensure to return in the original

When using @cwd %%FOO%%, we must ensure to return in the original
prefix later, but doing so with @cwd %%OLDPREFIX%% (having
PLIST_SUB+="OLDPREFIX=${PREFIX}") hardcodes the value in the packing
list. That's not really a problem when dealing with ports but that's
a problem with packages since pkg_add -p option only overrides the
first @cwd occurrence.

This patch allow us to use @cwd without any argument. If no
directory argument is given, it will set current working directory
to the first prefix given by the @cwd command.

PR:		bin/77212
Submitted by:	flz