Rename "linenumbers" to "enable-tree-linenumbers", change default to "1"

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>