This makes the name of the cgitrc option more descriptive and at the
same time changes the default from "0" to "1" in an attempt to stay
backwards compatible - prior to the introduction of "source-filter"
and "linenumbers", cgit always generated linenumber links in the
tree view, but now this feature can be turned off (one might want to
do this if the source-filter performs line-wrapping etc).
While at it, the documentation is updated to match the surrounding
descriptions.
Signed-off-by: Lars Hjemli <hjemli@gmail.com>