gpgconf: Show /proc/self/exe with -V and -X
* common/homedir.c (w32_rootdir): Factor some code out to ... (w32_myproc_self): here. (unix_rootdir): Factor some code out to ... (unix_myproc_self): here. (gnupg_myproc_self): New. * tools/gpgconf.c (show_version_gnupg): Use new function.
This is helpful for support in case several GnuPG versions are
installed.