rc: Fix kldxref script

[jlduran]

Summary

• Promote checking kldxref_clobber to a top-level guard, to avoid further processing if it is set to "NO" (the default)

• Check if the module directory contains "*.ko" files, if it does, generate the linker.hints file in the module directory

• Use $(...) notation instead of legacy backticks ...

Fixes: 2b55bea5380b ("Make /etc/rc.d/kldxref not print anything for directories that don't contain any kernel modules. This makes the common case completely silent, as it should be.")

[emaste]

I think this is fine. backticks and checkyesno moving are minor improvements (arguably not bugs).

I think the problem could also be fixed with [ "$(echo ${MODULE_DIR}/*.ko)" != "${MODULE_DIR}/*.ko" ] if we want to continue avoiding the extra process invocation. Is there a PR for it?

[jlduran]

In D49398#1126583, @emaste wrote:

I think this is fine. backticks and checkyesno moving are minor improvements (arguably not bugs).

I think the problem could also be fixed with [ "$(echo ${MODULE_DIR}/*.ko)" != "${MODULE_DIR}/*.ko" ] if we want to continue avoiding the extra process invocation. Is there a PR for it?

No, I will open one first. I'll abandon this revision.
I was working on some sh improvements, and this rc script was generating noise.