Replaced docs/COMPLETION-BASH with etc/bash_completions_nmh.
It's generated from man/mh-chart.man, which in turn is generated.
Automake doesn't like generated files in docs, hence the move to
etc. And the new filename better fits usage, I think.
Did not provide support for completing message numbers. It can
be fooled by programs that take multiple switches with +
arguments.
COMPLETION-BASH was broken with current bash, anyway.
Also, removed sbr/sigmsg.h from CLEANFILES in Makefile.am. It
was a leftover from commit
5776f9b783afe8e6bdbe430adb5d5b9435d25b19.