pgsql: Add BackendType for standalone backends

Add BackendType for standalone backends

All backends should have a BackendType to enable statistics reporting
per BackendType.

Add a new BackendType for standalone backends, B_STANDALONE_BACKEND (and
alphabetize the BackendTypes). Both the bootstrap backend and single
user mode backends will have BackendType B_STANDALONE_BACKEND.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion:
https://www.postgresql.org/message-id/CAAKRu_aaq33UnG4TXq3S-OSXGWj1QGf0sU%2BECH4tNwGFNERkZA%40mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/0c679464a837079acc75ff1d45eaa83f79e05690

Modified Files
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src/backend/utils/init/miscinit.c | 17 +++++++++++------
src/include/miscadmin.h           |  5 +++--
2 files changed, 14 insertions(+), 8 deletions(-)