Implement __sysremove for sim65

This will allow using unlink()/remove() in sim65 programs
Use it to unlink fgets' test output file

test/val/remove.c

@@ -0,0 +1,55 @@
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+int fails = 0;
+
+
+static void create_out_file(const char *outfile_path) {
+    FILE *out;
+
+
+    out = fopen(outfile_path, "wb");
+    if (out == NULL) {
+        printf("Could not create %s\n", outfile_path);
+        fails++;
+        return;
+    }
+    fclose(out);
+}
+
+int main (int argc, char **argv)
+{
+    int r;
+    static char outfile_path[FILENAME_MAX+1];
+
+    sprintf(outfile_path, "%s.test.out", argv[0]);
+
+    create_out_file(outfile_path);
+    r = remove(outfile_path);
+    if (r != 0) {
+        printf("could not remove() %s\n", outfile_path);
+        fails++;
+    }
+
+    create_out_file(outfile_path);
+    r = unlink(outfile_path);
+    if (r != 0) {
+        printf("could not unlink() %s\n", outfile_path);
+        fails++;
+    }
+
+    r = remove("klsdfjqlsjdflkqjdsoizu");
+    if (r == 0) {
+        printf("remove()ing non-existent file succeeded\n");
+        fails++;
+    }
+
+    r = unlink("klsdfjqlsjdflkqjdsoizu");
+    if (r == 0) {
+        printf("unlink()ing non-existent file succeeded\n");
+        fails++;
+    }
+
+    return fails;
+}