nrelease: clean up Makefile and sync man pages & Makefile.usr

nrelease: clean up Makefile and sync man pages & Makefile.usr

 * nrelease/Makefile: fix a few typos,
   change name of `syssrcs` target to `srcs`, to reflect current use and
   remove traces of old installer_* targets.
 * etc/Makefile: update name of src-sys tar file from nrelease and
   add targets for newly added src-all and pkgsrc-all tar files.
 * release.7: update to current method: only binary package downloaded is cvsup
   and add description of a few more variables.
 * development.7: add short description of /usr/Makefile.