Replace use of tr(1) with awk(1).

Unfortunately, GNU tr(1) is not Unicode-aware,
so we should use awk(1) instead. See
https://www.pixelbeat.org/docs/coreutils_i18n/ for more on the
situation of the support of Unicode in coreutils.

Apparently, awk is for this better than sed, because Unicode is
consistenly provided on all major versions of awk.

Signed-off-by: Matěj Cepl <mcepl@cepl.eu>
Supersedes: https://lists.sr.ht/~martanne/devel/patches/49113
Supersedes: https://lists.sr.ht/~martanne/devel/patches/49114

config.def.h

@@ -161,8 +161,8 @@ static const KeyBinding bindings_operators[] = {
 	{ "c",                  ACTION(OPERATOR_CHANGE)                     },
 	{ "d",                  ACTION(OPERATOR_DELETE)                     },
 	{ "g~",                 ALIAS(":|tr '[:lower:][:upper:]' '[:upper:][:lower:]'<Enter>") },
-	{ "gu",                 ALIAS(":|tr '[:upper:]' '[:lower:]'<Enter>")},
-	{ "gU",                 ALIAS(":|tr '[:lower:]' '[:upper:]'<Enter>")},
+	{ "gu",                 ALIAS(":|awk '{printf \"%s\", tolower($0)}'<Enter>")},
+	{ "gU",                 ALIAS(":|awk '{printf \"%s\", toupper($0)}'<Enter>")},
 	{ "p",                  ACTION(PUT_AFTER)                           },
 	{ "P",                  ACTION(PUT_BEFORE)                          },
 	{ "y",                  ACTION(OPERATOR_YANK)                       },