shellcheck: move common directives to .shellcheckrc

This simplifies our CI command line and makes it easier to document
expected usage (in HACKING.md).

There unfortunately isn't a way to set --checked-sourced or our default
warning level in .shellcheckrc, and our `find` command is still a bit
unwieldy, but this is still a net improvement.

.shellcheckrc

@@ -0,0 +1,8 @@
+# ShellCheck <https://www.shellcheck.net/>
+# https://github.com/koalaman/shellcheck/wiki/Directive#shellcheckrc-file
+
+# Allow opening any 'source'd file, even if not specified as input
+external-sources=true
+
+# Assume bash by default
+shell=bash