pgsql: Fix error handling of vacuumdb when running out of fds

Fix error handling of vacuumdb when running out of fds

When trying to use a high number of jobs, vacuumdb has only checked for
a maximum number of jobs used, causing confusing failures when running
out of file descriptors when the jobs open connections to Postgres.
This commit changes the error handling so as we do not check anymore for
a maximum number of allowed jobs when parsing the option value with
FD_SETSIZE, but check instead if a file descriptor is within the
supported range when opening the connections for the jobs so as this is
detected at the earliest time possible.

Also, improve the error message to give a hint about the number of jobs
recommended, using a wording given by the reviewers of the patch.

Reported-by: Andres Freund
Author: Michael Paquier
Reviewed-by: Andres Freund, Álvaro Herrera, Tom Lane
Discussion: https://postgr.es/m/20190818001858.ho3ev4z57fqhs7a5@alap3.anarazel.de
Backpatch-through: 9.5

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/eb91b8ee65721f7e4df218b7a438d9c55e7b972f

Modified Files
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src/bin/scripts/vacuumdb.c | 20 ++++++++++++++------
1 file changed, 14 insertions(+), 6 deletions(-)