Add proper errcodes to new error messages for read() failures

Those would use the default ERRCODE_INTERNAL_ERROR, but for foreseeable
failures an errcode ought to be set, ERRCODE_DATA_CORRUPTED making the
most sense here.

While on the way, fix one errcode_for_file_access missing in origin.c
since the code has been created, and remove one assignment of errno to 0
before calling read(), as this was around to fit with what was present
before 811b6e36 where errno would not be set when not enough bytes are
read.  I have noticed the first one, and Tom has pinged me about the
second one.

Author: Michael Paquier
Reported-by: Tom Lane
Discussion: https://postgr.es/m/27265.1531925836@sss.pgh.pa.us

src/backend/utils/cache/relmapper.c

@@ -669,7 +669,8 @@ load_relmap_file(bool shared)
 					 errmsg("could not read file \"%s\": %m", mapfilename)));
 		else
 			ereport(FATAL,
-					(errmsg("could not read file \"%s\": read %d of %zu",
+					(errcode(ERRCODE_DATA_CORRUPTED),
+					 errmsg("could not read file \"%s\": read %d of %zu",
 							mapfilename, r, sizeof(RelMapFile))));
 	}
 	pgstat_report_wait_end();