Default value for `procfs_files_spec` contains non-existing entries

The default value for `procfs_files_spec` should contain `/proc/loadavg` and `/proc/net/dev` as separate entries. Instead, it contains `/proc/loadavg/proc/net/dev`, which does not refer to any existing file, meaning both `/proc/loadavg` and `/proc/net/dev` are inaccessible.

Introduced at https://github.com/percona/percona-server/pull/4251/files#diff-e485ffe3e10db3bf4425b5224e2519e57332510fa327c4c6185bda4a5f98f113R121

The same issue occurs with the entry `/proc/self/numa_maps/proc/softirqs`, which should be `/proc/self/numa_maps` and `/proc/softirqs` respectively.

Introduced at https://github.com/percona/percona-server/pull/4251/files#diff-e485ffe3e10db3bf4425b5224e2519e57332510fa327c4c6185bda4a5f98f113R128