mmime: gen_qp: encode trailing space and tab as =20/=09

See RFC2045 6.7.(3):
> It follows that an octet with decimal value 9 or 32 appearing at the
> end of an encoded line must be represented according to Rule #1.

Prefer this over generating a soft-line break and then a real line break.

mmime.c

@@ -85,7 +85,6 @@ int gen_qp(uint8_t *s, off_t size, int maxlinelen, int linelen)
 		}
 
 		if ((s[i] > 126) ||
-		    (s[i] < 32 && s[i] != '\n' && s[i] != '\t') ||
 		    (s[i] == '=')) {
 			printf("=%02X", s[i]);
 			linelen += 3;
@@ -99,6 +98,18 @@ int gen_qp(uint8_t *s, off_t size, int maxlinelen, int linelen)
 			putc_unlocked('_', stdout);
 			linelen++;
 			prev = '_';
+		} else if (s[i] < 33 && s[i] != '\n') {
+			if ((s[i] == ' ' || s[i] == '\t') &&
+			    i+1 < size &&
+			    (s[i+1] != '\n' && s[i+1] != '\r')) {
+				putc_unlocked(s[i], stdout);
+				linelen += 1;
+				prev = s[i];
+			} else {
+				printf("=%02X", s[i]);
+				linelen += 3;
+				prev = '_';
+			}
 		} else if (s[i] == '\n') {
 			if (prev == ' ' || prev == '\t')
 				puts("=");