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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master

Details
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https://git.postgresql.org/pg/commitdiff/c9bd7f4f2b0fc3b8291914a45cbf2a5270877ee6

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