Add unlink() syscall and /bin/rm.

This patch adds most of the plumbing for working file deletion in Ext2FS.
Directory entries are removed and inode link counts updated.
We don't yet update the inode or block bitmaps, I will do that separately.

Userland/.gitignore

@@ -24,3 +24,4 @@ more
 guitest
 guitest2
 sysctl
+rm

Userland/Makefile

@@ -21,7 +21,8 @@ OBJS = \
        more.o \
        guitest.o \
        guitest2.o \
-       sysctl.o
+       sysctl.o \
+       rm.o
 
 APPS = \
        id \
@@ -47,7 +48,8 @@ APPS = \
        more \
        guitest \
        guitest2 \
-       sysctl
+       sysctl \
+       rm
 
 ARCH_FLAGS =
 STANDARD_FLAGS = -std=c++17 -nostdinc++ -nostdlib -nostdinc
@@ -139,6 +141,9 @@ guitest2: guitest2.o
 sysctl: sysctl.o
 	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
 
+rm: rm.o
+	$(LD) -o $@ $(LDFLAGS) $< ../LibC/LibC.a
+
 .cpp.o:
 	@echo "CXX $<"; $(CXX) $(CXXFLAGS) -o $@ -c $<
 

Userland/rm.cpp

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <errno.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+    if (argc != 2) {
+        fprintf(stderr, "usage: rm <path>\n");
+        return 1;
+    }
+    int rc = unlink(argv[1]);
+    if (rc < 0) {
+        perror("unlink");
+        return 1;
+    }
+    return 0;
+}
+