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-store/Makefile.am

@@ -1,17 +1,12 @@
 bin_PROGRAMS = nix-store
 
-nix_store_SOURCES =				\
-  nix-store.cc dotgraph.cc dotgraph.hh help.txt	\
+nix_store_SOURCES =			\
+  nix-store.cc dotgraph.cc dotgraph.hh	\
   xmlgraph.cc xmlgraph.hh
 
 nix_store_LDADD = ../libmain/libmain.la ../libstore/libstore.la ../libutil/libutil.la \
  ../boost/format/libformat.la -lbz2
 
-nix-store.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