patch 8.1.1384: using "int" for alloc() often results in compiler warnings

Problem:    Using "int" for alloc() often results in compiler warnings.
Solution:   Use "size_t" and remove type casts.  Remove alloc_check(), Vim
            only works with 32 bit ints anyway.
diff --git a/src/main.c b/src/main.c
index 79b4844..e1ddbf7 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2300,7 +2300,7 @@
 			}
 			else
 			    a = argv[0];
-			p = alloc((unsigned)(STRLEN(a) + 4));
+			p = alloc(STRLEN(a) + 4);
 			if (p == NULL)
 			    mch_exit(2);
 			sprintf((char *)p, "so %s", a);
@@ -2526,7 +2526,7 @@
      * one. */
     if (parmp->n_commands > 0)
     {
-	p = alloc((unsigned)STRLEN(parmp->commands[0]) + 3);
+	p = alloc(STRLEN(parmp->commands[0]) + 3);
 	if (p != NULL)
 	{
 	    sprintf((char *)p, ":%s\r", parmp->commands[0]);
@@ -4263,7 +4263,7 @@
 		size_t	len = STRLEN(cmd) + STRLEN(err) + 5;
 		char_u	*msg;
 
-		msg = alloc((unsigned)len);
+		msg = alloc(len);
 		if (msg != NULL)
 		    vim_snprintf((char *)msg, len, "%s: \"%s\"", err, cmd);
 		*result = msg;