install(1): Fix 2.5 y/o verbose bug, use static functions

install(1): Fix 2.5 y/o verbose bug, use static functions

A bug was introduced by this commit to add the -D option:

a2bc4e56d4ab068fea12d65d13740b7a600769a8 (7 SEP 11)

It changed the -v verbose option from standalone to expecting input and
as a result, any use of -v caused the usage screen to come up.  Fixed.
Additionally, the pathnames.h header is unused, removed now.
Finally, several functions were turned into static functions and explicit
void casts were used when functions normally return integers.  The last
two change sets converge with FreeBSD.

xinstall has not been seriously updated in a long time and has fallen
behind BSD counterparts.  Syncing with FreeBSD may take several steps as
there is significant divergence and options (e.g. -l) do different things
on FreeBSD