Correct a type in FindExt::apply_config in the "I18N-Langinfo" code.

Commit 557ab4cb986767c7 (Feb 2011) which added this routine had
$config->{i_nl_langinfo} not $config->{d_nl_langinfo} ('i' should be 'd').

The logic for I18N::Langinfo should now be correct.
FindExt::apply_config() needs to skip Sys::Syslog on Win32.

Without this, the test expectations on Win32 are inconsistent with what
actually got built.
Thanks to FindExt::apply_config() we're now able to test dynamic extensions.

FindExt::apply_config() mimic's Configure's logic, which means that FindExt's
idea of which extensions should be built is consistent with Configure's.
Minor refactors to FindExt's test, removing code duplication.

Use a ternary instead of if/unless on the same $^O test.
Use a loop to call FindExt::scan_ext()
As FindExt exports nothing, we can require it rather than using it.