LibC+userspace: Make everything compile with -Wall -Wextra -Werror

I added -Wall -Wextra -Werror as public compile flags to libc. Now
everything in userspace in compiled using these flags. I made all
necessary changes to allow compilation to work.

Only exception is execvp which has a large stack usage. Maybe it
should use malloc for the buffer but posix allows ENOMEM only when
kernel is out of memory... This can be fixed when fexecve is
implemented and there is no need for absolute path.

userspace/image/Netbpm.cpp

@@ -3,6 +3,7 @@
 #include <BAN/Optional.h>
 
 #include <ctype.h>
+#include <inttypes.h>
 #include <stdio.h>
 
 BAN::Optional<uint64_t> parse_u64(const uint8_t*& data, size_t data_size)
@@ -86,7 +87,7 @@ BAN::ErrorOr<BAN::UniqPtr<Image>> load_netbpm(const void* mmap_addr, size_t size
 		return BAN::Error::from_errno(EINVAL);
 	}
 
-	printf("Netbpm image %llux%llu\n", *width, *height);
+	printf("Netbpm image %" PRIuPTR "x%" PRIuPTR "\n", *width, *height);
 
 	BAN::Vector<Image::Color> bitmap;
 	TRY(bitmap.resize(*width * *height));