Replace backquotes

Replace backquoted form of command substitution with the "$()" form. Rationale: https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xcu_chap02.html#tag_23_02_06_03

Some of them were left untouched. For example, in debclean.sh:199:

-e '$re =~ s/PACKAGE/\\Q$pkg\\E/g; $pwd=`pwd`; chomp $pwd;'

Here, they're used in a Perl one-liner. In Perl, they're used in a similar context, though they can't be replaced with the () form because `(` is a special variable.