Fix some shell issues in etcupdate.sh

[bcr]

Summary

This patch does the following:

• Adds double quotes around variables to prevent globbing and word splitting, see SC2086
• Prefers $() over backticks in most cases, see SC2006
• Uses Use [ -n .. ] instead of ! [ -z .. ] logic, see SC2237

Test Plan

1. Apply patch to /usr.sbin/etcupdate
2. Check output or results when running it

[imp]

usr.sbin/etcupdate/etcupdate.sh

149

Surely quoting the whole root arg with one set of quotes?

"${DESTDIR}/root$1"

193–194

Same here: quote the entire arg

206–207

And here. May need ${1}

215

And here

228

And here and more below

[bcr]

Alright, I can provide a diff that does that. Makes it easier to read. I'll update this revision when it's ready.

[jhb]

Also, there are tests for etcupdate in usr.sbin/etcupdate/tests/

[bcr]

Updated diff addressing @bsdimp's comments. I changed the "$var"/"something" to "$var/something" to make it easier to read. When required, I can also wrap those variables to be ${var} if necessary.