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.