pgsql: Improve what pg_strsignal prints if we haven't gotstrsignal(3).

Improve what pg_strsignal prints if we haven't got strsignal(3).

Turns out that returning "unrecognized signal" is confusing.
Make it explicit that the platform lacks any support for signal names.
(At least of the machines in the buildfarm, only HPUX lacks it.)

Back-patch to v12 where we invented this function.

Discussion: https://postgr.es/m/3067.1566870481@sss.pgh.pa.us

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7570df0f3b6d873cd060b3271b5f7b90cbcfb66c

Modified Files
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src/port/pgstrsignal.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)