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.