tree-wide: replace "plural(s)" by "plurals"

(s) is just ugly with a vibe of DOS. In most cases just using the normal plural
form is more natural and gramatically correct.

There are some log_debug() statements left, and texts in foreign licenses or
headers. Those are not touched on purpose.

src/basic/meson.build

@@ -389,7 +389,7 @@ filesystem_includes = ['linux/magic.h',
 check_filesystems = find_program('check-filesystems.sh')
 r = run_command([check_filesystems, cpp, files('filesystems-gperf.gperf')] + filesystem_includes, check: false)
 if r.returncode() != 0
-        error('found unknown filesystem(s) defined in kernel headers:\n\n' + r.stdout())
+        error('Unknown filesystems defined in kernel headers:\n\n' + r.stdout())
         r.stdout()
 endif