save-cwd: reduce default dependency

save-cwd: reduce default dependency

save-cwd generally needs only a working fchdir or a working
getcwd(NULL,0).  If you are not worried about directories whose
absolute name is longer than PATH_MAX, then reducing the default
dependencies reduces the bulk for this module.

However, there are cases where neither function works on Linux
(an unreadable but searchable directory can be opened by O_SEARCH,
except that Linux doesn't implement that yet; and Linux getcwd()
has issues with long absolute names which glibc does not work
around but which the full-blown getcwd module does).  So someone
desiring a truly robust solution needs to import the 'getcwd'
module at the same time as 'save-cwd'.

* modules/save-cwd (Depends-on): Use getcwd-lgpl.
* lib/save-cwd.c: Update comments.
* NEWS: Document the semantic change.

Signed-off-by: Eric Blake <eblake@redhat.com>