obj_mac.h is missing #include guards, so one cannot use NIDs without
pulling in the OBJ_* functions which depend on the giant OID table. Give
it #include guards, tidy up the style slightly, and also rename it to
nid.h which is a much more reasonable name.
obj_mac.h is kept as a forwarding header as, despite it being a little
screwy, some code #includes it anyway.
BUG=chromium:499653
Change-Id: Iec0b3f186c02e208ff1f7437bf27ee3a5ad004b7
Reviewed-on: https://boringssl-review.googlesource.com/7562
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
