main/mksh: add and fall back on /etc/mkshrc

fixes #12398
7 jobs for !19173 with patch-mksh in 2 minutes and 15 seconds
latest detached
Status Job ID Name Coverage
  Lint
passed #342891
docker-alpine x86_64
lint

00:00:18

 
  Build
passed #342896
docker-alpine ci-build aarch64
build-aarch64

00:01:37

passed #342897
docker-alpine ci-build armv7
build-armv7

00:01:46

passed #342895
docker-alpine ci-build ppc64le
build-ppc64le

00:01:53

passed #342894
docker-alpine ci-build s390x
build-s390x

00:01:40

passed #342893
docker-alpine ci-build x86
build-x86

00:01:03

passed #342892
docker-alpine ci-build x86_64
build-x86_64

00:01:01