List of all link-set names.
libdm is needed by LVM early in the boot process. Fixes bin/49357.
Synchronise the "-s" part of the usage message with postinstall.
A colon-separated list of tgz files is no longer accepted; you
must use multiple -s options instead.
catch up with current ISO naming conventions
Document "extsrc".  This was added to src/BUILDING in revision 1.90
dated 2009-11-30, but was not added to src/doc/BUILDING.mdoc,
so the changes were lost.

The original log message was:
- - - -
revision 1.90
date: 2009-11-30 16:13:22 +0000;  author: uebayasi;  state: Exp;  lines: +22 -0;
Support "extsrc", externally added programs and libraries.  Users can write
their own reach-overs, cross-build, install, and get set files just like base
and X11 / X.org.  (These sets are not included as TNF releases.)
- - - -
regen from doc/BUILDING.mdoc revision 1.107.
This reinstates documentation for "extsrc" which was lost in 2009.