Readd gpgwrap as non-console wrapper

* src/Makefile.am: Build it.
* src/inst-gnupg-w32.nsi: Install gpgwrap under -w32.exe suffix
* src/uninst-gnupg-w32.nsi: Uninstall.
* srC/gpgwrap.c: Readded.

The idea is to have a generic wrapper linked with subsystem
windows to make it easier to call gpg from scripts without
opening a console window.

This is based on the old gpgwrap but removes the path
changing and instead changes the suffix of the binary.