fsfreeze: Add tool

Add fsfreeze, a Linux-specific tool used to freeze/thaw writes to a
filesystem. It's an extremely simple tool so this should have full
feature-parity.

Cargo.lock

@@ -1022,6 +1022,7 @@ dependencies = [
  "textwrap",
  "uu_ctrlaltdel",
  "uu_dmesg",
+ "uu_fsfreeze",
  "uu_last",
  "uu_lscpu",
  "uu_lsmem",
@@ -1053,6 +1054,17 @@ dependencies = [
  "uucore",
 ]
 
+[[package]]
+name = "uu_fsfreeze"
+version = "0.0.1"
+dependencies = [
+ "clap",
+ "linux-raw-sys 0.6.5",
+ "regex",
+ "sysinfo",
+ "uucore",
+]
+
 [[package]]
 name = "uu_last"
 version = "0.0.1"