Makefile.am: pass D variable to shell scripts' environment

Makefile.am: pass D variable to shell scripts' environment

And set -x if D is set to 1, so the commands called by various shell scripts
are easier to see during debugging.

* Makefile.am (bpf_attr_check.c, sen.h, mpers-m%.stamp, $(srcdir)/CREDITS): Pass
D variable in the environment to the shell script call.
* gen_bpf_attr_check.sh: Set -x if D is set to 1.
* generate_mpers_am.sh: Likewise.
* generate_sen.sh: Likewise.
* maint/gen-contributors-list.sh: Likewise.
* mpers.sh: Likewise.
* mpers_test.sh: Likewise.