Add a meson option for a local public suffix list

gen-public-suffix.py downloads public_suffix_list.dat at build time and
only falls back to a few hardcoded Linux paths when that fails. Package
builds usually run without network access, and on FreeBSD the list is
installed under /usr/local/share/public_suffix_list, so the build dies
with "unable to load public suffix list".

Add a public-suffix-list option that takes the path of a local copy.
When it is set the generator reads just that file and never goes
online, which also keeps the generated table reproducible.

meson_options.txt

@@ -72,3 +72,8 @@ option('with-upd', type: 'boolean',
 option('with-perl-legacy-api', type: 'boolean', value: false,
   description: 'Enables the legacy IRC perl module for compatibility with old scripts'
 )
+
+# Build inputs
+option('public-suffix-list', type: 'string', value: '',
+  description: 'Path to a local public_suffix_list.dat to generate the URL detection rules from instead of downloading it'
+)