When ‘--help’ is given, just run ‘man’ to show the manual page

I.e. do what git does.  I'm too lazy to keep the builtin help text up
to date :-)

Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).

src/nix-hash/Makefile.am

@@ -1,13 +1,8 @@
 bin_PROGRAMS = nix-hash
 
-nix_hash_SOURCES = nix-hash.cc help.txt
+nix_hash_SOURCES = nix-hash.cc
 nix_hash_LDADD = ../libmain/libmain.la ../libstore/libstore.la ../libutil/libutil.la \
  ../boost/format/libformat.la
 
-nix-hash.o: help.txt.hh
-
-%.txt.hh: %.txt
-	../bin2c/bin2c helpText < $< > $@ || (rm $@ && exit 1)
-
 AM_CXXFLAGS = \
  -I$(srcdir)/.. -I$(srcdir)/../libutil -I$(srcdir)/../libstore -I$(srcdir)/../libmain