config: Move (u|g)idMappings from 'process' to 'linux'
config: Move (u|g)idMappings from 'process' to 'linux'

To match where they're defined in the JSON Schema [1].  The old
location is from d4e7326d (config: JSON examples, 2016-04-06, #370),
and seems to have been accidental.

[1]: https://github.com/opencontainers/runtime-spec/blob/0982071b288ddddc1ae84d21c4bd682c96942f5c/schema/schema-linux.json#L21-L48

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

ujuc/specs,wking/opencontainer-runtime-spec,wking/opencontainer-specs,cloudfoundry-incubator/specs,laijs/specs,jonboulle/specs,opencontainers/specs,hqhq/specs,hqhq/runtime-spec,vbatts/specs,opencontainers/runtime-spec