Assign "backend" type earlier during process start-up

Assign "backend" type earlier during process start-up

Instead of assigning the backend type in the Main function of each
postmaster child, do it right after fork(), by which time it is already
known by postmaster_child_launch().  This reduces the time frame during
which MyBackendType is incorrect.

Before this commit, ProcessStartupPacket would overwrite MyBackendType
to B_BACKEND for dead-end backends, which is quite dubious.  Stop that.

We may now see MyBackendType == B_BG_WORKER before setting up
MyBgworkerEntry.  As far as I can see this is only a problem if we try
to log a message and %b is in log_line_prefix, so we now have a constant
string to cover that case.  Previously, it would print "unrecognized",
which seems strictly worse.

Author: Euler Taveira <euler@eulerto.com>
Discussion: https://postgr.es/m/e85c6671-1600-4112-8887-f97a8a5d07b2@app.fastmail.com