Initial rewrite of config as a Perl module

- Use $^X; to find perl.
- Big re-ordering: Put all variables at the top, move most inline code into
  functions. The heart of the script now basically just calls
  functions to do its work.
- Unify warning text, add -w option
- Don't use needless (subshells)
- Ensure Windows gets a VC-xxx option
- Make config a perl module
- Top-level "config" command-line is a dummy that just calls the module.
  Added module stuff so that it can be called from Configure.
Reviewed-by: NTim Hudson <tjh@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11230)