pgsql: Add support for abstract Unix-domain sockets

Add support for abstract Unix-domain sockets

This is a variant of the normal Unix-domain sockets that don't use the
file system but a separate "abstract" namespace. At the user
interface, such sockets are represented by names starting with "@".
Supported on Linux and Windows right now.

Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://www.postgresql.org/message-id/flat/6dee8574-b0ad-fc49-9c8c-2edc796f0033(at)2ndquadrant(dot)com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c9f0624bc2f544baacafa38e3797d5323401d039

Modified Files
--------------
doc/src/sgml/config.sgml | 24 +++++++++++++++++++++++-
doc/src/sgml/libpq.sgml | 5 ++++-
src/backend/libpq/pqcomm.c | 8 ++++++++
src/bin/psql/command.c | 15 +++++----------
src/bin/psql/prompt.c | 3 ++-
src/common/ip.c | 24 +++++++++++++++++++++++-
src/include/libpq/pqcomm.h | 9 +++++++++
src/interfaces/libpq/fe-connect.c | 4 ++--
8 files changed, 76 insertions(+), 16 deletions(-)