pgsql: Get rid of the dedicated latch for signaling the startup process

Get rid of the dedicated latch for signaling the startup process.

This commit gets rid of the dedicated latch for signaling the startup
process in favor of using its procLatch,  since that comports better
with possible generic signal handlers using that latch.

Commit 1e53fe0e70 changed background processes so that they use standard
SIGHUP handler. Like that, this commit also makes the startup process use
standard SIGHUP handler to simplify the code.

Author: Fujii Masao
Reviewed-by: Bharath Rupireddy, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACXPorUqePswDtOeM_s82v9RW32E1fYmOPZ5NuE+TWKj_A@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ac22929a2613e122708bd0172508ac863c51c1cc

Modified Files
--------------
src/backend/access/transam/xlog.c | 29 ++++++++++++++---------------
src/backend/postmaster/startup.c  | 24 +++++-------------------
2 files changed, 19 insertions(+), 34 deletions(-)