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/quickfix.c b/src/quickfix.c
index cf3b274..46eae2d 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -1346,7 +1346,7 @@
 	if (*fields->errmsg && !qfl->qf_multiignore)
 	{
 	    len = (int)STRLEN(qfprev->qf_text);
-	    if ((ptr = alloc((unsigned)(len + STRLEN(fields->errmsg) + 2)))
+	    if ((ptr = alloc(len + STRLEN(fields->errmsg) + 2))
 		    == NULL)
 		return QF_FAIL;
 	    STRCPY(ptr, qfprev->qf_text);
@@ -1890,7 +1890,7 @@
 {
     qf_delq_T	*q;
 
-    q = (qf_delq_T *)alloc((unsigned)sizeof(qf_delq_T));
+    q = (qf_delq_T *)alloc(sizeof(qf_delq_T));
     if (q != NULL)
     {
 	q->qi = qi;
@@ -2063,7 +2063,7 @@
     qfline_T	*qfp;
     qfline_T	**lastp;	// pointer to qf_last or NULL
 
-    if ((qfp = (qfline_T *)alloc((unsigned)sizeof(qfline_T))) == NULL)
+    if ((qfp = (qfline_T *)alloc(sizeof(qfline_T))) == NULL)
 	return QF_FAIL;
     if (bufnum != 0)
     {
@@ -2429,7 +2429,7 @@
     struct dir_stack_T  *ds_ptr;
 
     // allocate new stack element and hook it in
-    ds_new = (struct dir_stack_T *)alloc((unsigned)sizeof(struct dir_stack_T));
+    ds_new = (struct dir_stack_T *)alloc(sizeof(struct dir_stack_T));
     if (ds_new == NULL)
 	return NULL;
 
@@ -4707,7 +4707,7 @@
 	else
 	    off += 19;
 
-	name = alloc((unsigned)STRLEN(p_mef) + 30);
+	name = alloc(STRLEN(p_mef) + 30);
 	if (name == NULL)
 	    break;
 	STRCPY(name, p_mef);