common: Add a global variable to for the default error source.

For the shared code parts it is cumbersome to pass an error sourse
variable to each function.  Its value is always a constant for a given
binary and thus a global variable makes things a lot easier than the
former macro stuff.
* common/init.c (default_errsource): New global var.
(init_common_subsystems): Rename to _init_common_subsystems.  Set
DEFAULT_ERRSOURCE.
* common/init.h: Assert value of GPG_ERR_SOURCE_DEFAULT.
(init_common_subsystems): New macro.
* common/util.h (default_errsource): Add declaration.
* kbx/keybox-defs.h: Add some GPG_ERR_SOURCE_DEFAULT trickery.

dirmngr/dirmngr.c

@@ -56,6 +56,7 @@
 #include "ldapserver.h"
 #include "asshelp.h"
 #include "ldap-wrapper.h"
+#include "../common/init.h"
 
 /* The plain Windows version uses the windows service system.  For
    example to start the service you may use "sc start dirmngr".

dirmngr/dirmngr_ldap.c

@@ -57,6 +57,7 @@
 
 #include "i18n.h"
 #include "util.h"
+#include "../common/init.h"
 
 /* With the ldap wrapper, there is no need for the npth_unprotect and leave
    functions; thus we redefine them to nops.  If we are not using the