utils: add -h,--help command line flag to some utilities and update manpages

Added either -h or --help to gsymfix, refdes_renum and garchive and
updated the respective manpages. Also some editing for formatting while
I was in the manpages.

Closes-bug: lp-1218833
gschem: Change selection rules to make it easier to deselect nets.

gschem has a feature for selecting all connected net segments on a
sheet.  It works by looking for a click on a selected net segment in
select mode.  Unfortunately, this prevented deselection of a net
segment using Shift+<mouse-1> or Ctrl+<mouse-1> if that net segment
has any connected, unselected net segment.

This patch prevents selection of connected net segments if a modifier
key is pressed.

Closes-bug: lp-1227903
gschemdoc: Search user-specific documentation directory.

Enhance gschemdoc to search in $XDG_DATA_HOME/doc/geda-gaf (usually
$HOME/.local/share/doc/geda-gaf) before looking in the system
documentation directory.

Closes-bug: lp-971585
gschem: Remove some compatibility code for GTK+ < 2.6.
docs: Remove some unused & duplicate files from wiki export.

Closes-bug: lp-1081306