std.getopt: Make private functions safe.

[jondegenhardt]

This change make a pair of internal std.getopt functions @safe. This enables some additional @safe getopt use: callbacks via delegates and config option setting. @safe unit tests for both are included.

I also went through all the existing unit tests to see if any could be converted from @system to @safe. However, the existing @system unit tests take the address of local variables are cannot be @safe.

This PR is a follow-up to PR #5347.

[JackStouffer]

@nogc pure nothrow should be added as well if applicable

[quickfur]

LGTM, thanks!

[quickfur]

And yes, if pure, nothrow, etc., are applicable, they should be added as well (to unittests and non-template functions only, of course).

[jondegenhardt]

Added pure nothrow @nogc to setConfig. Also changed it to use final switch. However, optMatch calls functions not yet marked, so not none of these attributes could be applied to it. Also means none of the unit tests can be marked with these attributes.

[CyberShadow]

This pull request introduced a regerssion:
https://issues.dlang.org/show_bug.cgi?id=17650