treewide: fix "an" before consonant U sounds

The article "a" goes before consonant sounds and "an" goes before vowel
sounds. This commit changes an to a for UKI, UDP, UTF-8, URL, UUID, U-Label, UI
and USB, since they start with the sound /ˌjuː/.

src/shared/ask-password-api.c

@@ -197,7 +197,7 @@ static int backspace_string(int ttyfd, const char *str) {
         size_t m = utf8_n_codepoints(str);
         if (m == SIZE_MAX)
                 m = strlen(str); /* Not a valid UTF-8 string? If so, let's backspace the number of bytes
-                                  * output. Most likely this happened because we are not in an UTF-8 locale,
+                                  * output. Most likely this happened because we are not in a UTF-8 locale,
                                   * and in that case that is the correct thing to do. And even if it's not,
                                   * terminals tend to stop backspacing at the leftmost column, hence
                                   * backspacing too much should be mostly OK. */

src/shared/dns-domain.c

@@ -301,7 +301,7 @@ int dns_label_apply_idna(const char *encoded, size_t encoded_size, char *decoded
         assert(encoded);
         assert(decoded);
 
-        /* Converts an U-label into an A-label */
+        /* Converts a U-label into an A-label */
 
         r = dlopen_idn();
         if (r < 0)
@@ -353,7 +353,7 @@ int dns_label_undo_idna(const char *encoded, size_t encoded_size, char *decoded,
         size_t w;
         int r;
 
-        /* To be invoked after unescaping. Converts an A-label into an U-label. */
+        /* To be invoked after unescaping. Converts an A-label into a U-label. */
 
         assert(encoded);
         assert(decoded);

src/shared/pretty-print.c

@@ -42,7 +42,7 @@ int terminal_urlify(const char *url, const char *text, char **ret) {
 
         assert(url);
 
-        /* Takes an URL and a pretty string and formats it as clickable link for the terminal. See
+        /* Takes a URL and a pretty string and formats it as clickable link for the terminal. See
          * https://gist.github.com/egmontkob/eb114294efbcd5adb1944c9f3cb5feda for details. */
 
         if (isempty(text))

src/shared/qrcode-util.c

@@ -93,7 +93,7 @@ int print_qrcode(FILE *out, const char *header, const char *string) {
         QRcode* qr;
         int r;
 
-        /* If this is not an UTF-8 system or ANSI colors aren't supported/disabled don't print any QR
+        /* If this is not a UTF-8 system or ANSI colors aren't supported/disabled don't print any QR
          * codes */
         if (!is_locale_utf8() || !colors_enabled())
                 return -EOPNOTSUPP;