Abbreviate long field descriptors at write time so that they don't get
clobbered at runtime:

dirtybuf -> dtbuf
desiredvnodes -> desvn
numvnodes -> numvn
freevnodes -> frevn

The vmstats column has only 5 characters available for descriptors, but up
to 13 were used. The extras get clobbered at runtime by interrupt values
and/or descriptors on systems with more than 12 interrupt sources.

%slo-z -> %sloz

This one is in the "extended" vmstats area and doesn't get clobbered now.

Removed stale documentation of desvn.

Changed a descriptor:

tfree -> totfr

so that it is consistent with the abbreviations for other free counts
(daefr and prcfr) and thus almost decodeable.

Fixed missing documentation of tfree/totfr. This and everything else
in the extended vmstats area is misdocumented as being in a certain
place in the vmstats column.