Flush the buffercache to 16MB on hibernate and restore its previous max
size (kern.bufcachepercent) on resume, for better hibernate performance.

ok beck@
Add basic DWARF line table decoder

Includes a stand-alone addr2line clone for userspace testing.

Tested extensively on amd64 and expected to eventually support other
architectures too.  Importing now so further development/testing can
happen in-tree.  Followup commits will add to the kernel build and
integrate into ddb.

positive feedback; no objections
Allow ftp(1) to change its User-Agent for HTTP(S) URL requests using a
-U command-line option.

feedback from deraadt@, halex@, and Adam Thompson
ok deraadt@ sthen@, man page changes ok jmc@