pgsql: Add port/ replacement for strsep()

Add port/ replacement for strsep()

from OpenBSD, similar to strlcat, strlcpy

There are currently no uses, but some will be added soon.

Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: David Steele <david@pgmasters.net>
Discussion: https://www.postgresql.org/message-id/flat/79692bf9-17d3-41e6-b9c9-fc8c3944222a@eisentraut.org

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/683be87fbba02b086cc05789fcbdc289120d1065

Modified Files
--------------
configure                  | 23 ++++++++++++++
configure.ac               |  3 +-
meson.build                |  2 ++
src/include/pg_config.h.in |  7 +++++
src/include/port.h         |  4 +++
src/port/meson.build       |  1 +
src/port/strsep.c          | 77 ++++++++++++++++++++++++++++++++++++++++++++++
7 files changed, 116 insertions(+), 1 deletion(-)