added web mode; require root only only for repo and source mode; adjusted files install attributes in Makefile

The web mode brought a lot of changes. First of all the web service should *not*
run as root, thus requireing root priviledges for from main had to be dropped.
And since some modes can be used without being root, they are now allowed however
the repo and source mode can not be used at all, not even the help messages are
shown (on purpose).

Second, spm has moved to /bin (by default) from /sbin (by default) so that users
can executed it now that root is not always required.

The third thing, introduced LOG_FILE so that while in web mode messages are
logged to different file and logs are not messed up.

On a side note, install-web target (in Makefile) is introduced for the resources
needed from the web mode to run and serve properly. Files are in /opt/spm/web
altough /usr/share/spm may be a good option too.