Bug#42799: update-alternatives dies renaming across filesystems

[Drew Bloechl <drew@varesearch.com>]

Package: dpkg
Version: 1.4.1.6

While I was upgrading gcc, update-alternatives bombed out.  /usr/share 
and /usr are different filesystems on my machine:

bottomless:~# df -h /usr /usr/share
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda7             6.9G  1.8G  4.8G  27% /usr
/dev/sdb1             8.2G  4.0G  3.7G  52% /usr/share

bottomless:~# dpkg --configure gcc
Setting up gcc (2.95-3) ...
Checking available versions of cc, updating links in /etc/alternatives ...
(You may modify the symlinks there yourself if desired - see `man ln'.)
Renaming cc.1.gz slave link from /usr/man/man1/cc.1.gz to /usr/share/man/man1/cc.1.gz.
update-alternatives: unable to rename /usr/man/man1/cc.1.gz to /usr/share/man/man1/cc.1.gz: Invalid cross-device link
dpkg: error processing gcc (--configure):
 subprocess post-installation script returned error exit status 2
Errors were encountered while processing:
 gcc