[libc][string] Add strcasecmp support

Add strcasecmp() for case insensitive string compares. There is no ISO
standard function for this purpose but strcasecmp() is POSIX standard.

lib/libc/include/string.h

@@ -21,6 +21,7 @@ void *memset (void *, int, size_t);
 char       *strcat(char *, char const *);
 char       *strchr(char const *, int) __PURE;
 int         strcmp(char const *, char const *) __PURE;
+int         strcasecmp(char const *, char const *) __PURE;
 char       *strcpy(char *, char const *);
 char       *strerror(int) __CONST;
 size_t      strlen(char const *) __PURE;

lib/libc/string/rules.mk

@@ -8,6 +8,7 @@ C_STRING_OPS := \
 	memcpy \
 	memmove \
 	memset \
+	strcasecmp \
 	strcat \
 	strchr \
 	strcmp \

lib/libc/string/strcasecmp.c

@@ -0,0 +1,28 @@
+/*
+** Copyright 2001, Travis Geiselbrecht. All rights reserved.
+** Distributed under the terms of the NewOS License.
+*/
+/*
+ * Copyright (c) 2008 Travis Geiselbrecht
+ * Copyright 2022 Google LLC
+ *
+ * Use of this source code is governed by a MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT
+ */
+#include <ctype.h>
+#include <string.h>
+#include <sys/types.h>
+
+int strcasecmp(char const *cs, char const *ct) {
+    signed char __res;
+
+    while (1) {
+        if ((__res = tolower(*cs) - tolower(*ct)) != 0 || !*cs)
+            break;
+        cs++;
+        ct++;
+    }
+
+    return __res;
+}