New Windows API gpgrt_w32_reg_get_string.

* src/w32-reg.c (_gpgrt_w32_reg_query_string): Implement a prefix for
root and consider an empty string for root also HKCU.
* src/visibility.c (gpgrt_w32_reg_get_string): New.
* src/gpg-error.def.in: Add new function.
* src/gpg-error.c (my_strusage): Print the Windows version (32 or 64
bit) of the binary.
(main): New command --getreg.

This function is already used internally and we have a similar
function in GnuPG. It is much more convenient to work with this
function.

Also we can now pass the KEY_WOW64_32KEY andKEY_WOW64_64KEY flags to
the registry read functions. For example

!32key!HKLM\software\gnu\gnupg:ntdskeyserver

returns the value using the KEY_WOW64_32KEY flag; likewise for
!64key!. If !cross! is used the reversed flag of the binary type is
used.