patch 8.1.1414: alloc() returning "char_u *" causes a lot of type casts

Problem:    Alloc() returning "char_u *" causes a lot of type casts.
Solution:   Have it return "void *". (Mike Williams)  Define ALLOC_ONE() to
            check the simple allocations.
diff --git a/src/fileio.c b/src/fileio.c
index ee8c14d..d746298 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -6533,7 +6533,7 @@
 	return -1;
     }
 
-    buffer = (char *)alloc(BUFSIZE);
+    buffer = alloc(BUFSIZE);
     if (buffer == NULL)
     {
 	close(fd_out);
@@ -6890,8 +6890,7 @@
 	{
 	    if (!helpmesg)
 		mesg2 = "";
-	    tbuf = (char *)alloc(STRLEN(path) + STRLEN(mesg)
-							  + STRLEN(mesg2) + 2);
+	    tbuf = alloc(STRLEN(path) + STRLEN(mesg) + STRLEN(mesg2) + 2);
 	    sprintf(tbuf, mesg, path);
 #ifdef FEAT_EVAL
 	    /* Set warningmsg here, before the unimportant and output-specific