Previously the shutdown of a background worker that uses die() as
SIGTERM signal handler produced the log message "terminating
connection due to administrator command". This log message was
confusing because a background worker is not a connection.
This commit improves that log message to "terminating background
worker XXX due to administrator command" (XXX is replaced with
the name of the background worker). This is the same log message
as another SIGTERM signal handler bgworker_die() for a background
worker reports.

Author: Bharath Rupireddy
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/3f292fbb-f155-9a01-7cb2-7ccc9007ab3f@oss.nttdata.com