xmalloc: remove delim param from xstrjoin() and add separate xstrjoin3()

This avoids the need for an unused third argument for most xstrjoin()
calls and replaces the cases where it's needed with a more flexible
function. Code generation is the same in both cases, when there are 2
string params and a compile-time known delimiter.

This commit also converts 4 uses of xasprintf() to use xstrjoin*().

See also: https://godbolt.org/z/xsjrhv9b6

xmalloc.h

@@ -25,16 +25,25 @@ xmemdup(const void *ptr, size_t size)
 }
 
 static inline char *
-xstrjoin(const char *s1, const char *s2, char delim)
+xstrjoin(const char *s1, const char *s2)
 {
     size_t n1 = strlen(s1);
-    size_t n2 = delim > 0 ? 1 : 0;
-    size_t n3 = strlen(s2);
-
-    char *joined = xmalloc(n1 + n2 + n3 + 1);
+    size_t n2 = strlen(s2);
+    char *joined = xmalloc(n1 + n2 + 1);
     memcpy(joined, s1, n1);
-    if (delim > 0)
-        joined[n1] = delim;
-    memcpy(joined + n1 + n2, s2, n3 + 1);
+    memcpy(joined + n1, s2, n2 + 1);
+    return joined;
+}
+
+static inline char *
+xstrjoin3(const char *s1, const char *s2, const char *s3)
+{
+    size_t n1 = strlen(s1);
+    size_t n2 = strlen(s2);
+    size_t n3 = strlen(s3);
+    char *joined = xmalloc(n1 + n2 + n3 + 1);
+    memcpy(joined, s1, n1);
+    memcpy(joined + n1, s2, n2);
+    memcpy(joined + n1 + n2, s3, n3 + 1);
     return joined;
 }