Add support for `NUT_DEFAULT_CONNECT_TIMEOUT` envvar in Python/Perl/... client bindings

[jimklimov]

From brewing changes in PR #2847, tying envvars/defaults/configs/cli into one timeout sort of works:

$ NUT_DEFAULT_CONNECT_TIMEOUT=z3.8 NUT_DEBUG_LEVEL=6 ./clients/upsc eco650
   0.000000     [D1:30850] Starting NUT client: upsc
   0.000000     [D1:30850] upscli_init_default_timeout: NUT_DEFAULT_CONNECT_TIMEOUT='z3.8' value was not recognized, ignored
   0.000030     [D1:30850] upscli_init_default_timeout: upscli_default_timeout=10.000000 sec assigned from: default_secs

$ NUT_DEFAULT_CONNECT_TIMEOUT=3.8 NUT_DEBUG_LEVEL=6 ./clients/upsc eco650
   0.000000     [D1:30701] Starting NUT client: upsc
   0.000000     [D1:30701] upscli_init_default_timeout: upscli_default_timeout=3.800000 sec assigned from: envvar_secs

$ NUT_DEFAULT_CONNECT_TIMEOUT=z3.8 NUT_DEBUG_LEVEL=6 ./clients/upsc -W 18.25 eco650
   0.000000     [D1:31283] Starting NUT client: upsc
   0.000001     [D1:31283] upscli_init_default_timeout: NUT_DEFAULT_CONNECT_TIMEOUT='z3.8' value was not recognized, ignored
   0.000030     [D1:31283] upscli_init_default_timeout: upscli_default_timeout=18.250000 sec assigned from: cli_secs

Originally posted by @jimklimov in #2847 (comment)