Don't attempt to read or write ~/.lesshst if it's not a regular file
or a symlink to a regular file. Previously, symlinking to /dev/null
would cause less to trash /dev/null if run with sufficient privileges,
as seen in PR misc/42237.

While the official way to disable .lesshst is to set an environment
variable, that is problematic in some cases, such as single-user mode.
A safer way to prevent even an unpatched less from writing anything
out is to mkdir ~/.lesshst.

(dholland)