Tweak startup sequence so that running out of PROC array slots is

Tweak startup sequence so that running out of PROC array slots is
detected sooner in backend startup, and is treated as an expected error
(it gives 'Sorry, too many clients already' now).  This allows us not
to have to enforce the MaxBackends limit exactly in the postmaster.
Also, remove ProcRemove() and fold its functionality into ProcKill().
There's no good reason for a backend not to be responsible for removing
its PROC entry, and there are lots of good reasons for the postmaster
not to be touching shared-memory data structures.