genserv.pl: make external calls safe

[vszakats]

By passing command-line as separate arguments instead of using a single
string. This needs skipping the shell, so rework redirections to use
Perl open3().

Also explored to use -out to avoid redirections, but it makes the
command-line incompatible with some OpenSSL implementations/versions
(e.g. on default macOS), and would still need a solution for
2>/dev/null.

Ref: https://perldoc.perl.org/IPC::Open3

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• make the 5 calls continue to redirect stderr to /dev/null