[6s] Allow 6s to know about filesystems

Added a j6_proto_vfs_tag/_get_tag pair of messages to the VFS protocol
to allow filesystems to label themselves, and gave 6s the concept of
current fs and current working directory.

src/user/6s/commands.cpp

@@ -1,23 +1,45 @@
 #include <edit/line.h>
 #include <util/format.h>
 #include "commands.h"
+#include "shell.h"
 
 static void
-help(edit::source &s)
+fslist(shell &s)
 {
+    char line[100];
+    util::counted<char> output { line, sizeof(line) };
+
+    for (auto &f : s.filesystems()) {
+        size_t len = util::format(output, "%s\r\n", f.tag);
+        s.write(line, len);
+    }
+}
+
+static void
+help(shell &s)
+{
+    char line[100];
+    util::counted<char> output { line, sizeof(line) };
+
     for (unsigned i = 0; i < g_builtins_count; ++i) {
-        char line[100];
-        util::counted<char> output { line, sizeof(line) };
-
         builtin &cmd = g_builtins[i];
-
         size_t len = util::format(output, "%20s - %s\r\n", cmd.name(), cmd.description());
         s.write(line, len);
     }
 }
 
 static void
-version(edit::source &s)
+pwd(shell &s)
+{
+    char line[100];
+    util::counted<char> output { line, sizeof(line) };
+
+    size_t len = util::format(output, "%s:%s\r\n", s.cfs(), s.cwd());
+    s.write(line, len);
+}
+
+static void
+version(shell &s)
 {
     static const char *gv = GIT_VERSION;
 
@@ -29,7 +51,9 @@ version(edit::source &s)
 }
 
 builtin g_builtins[] = {
-    { "help",       "list availble commands",               help },
+    { "fslist",     "list available filesystems",           fslist },
+    { "help",       "list available commands",              help },
+    { "pwd",        "print current working directory",      pwd },
     { "version",    "print current jsix version",           version },
 };
 size_t g_builtins_count = sizeof(g_builtins) / sizeof(g_builtins[0]);