Add crypt.[ch] and update md5sum and sha1sum

Factor out the code from md5sum and sha1sum into a util function.

Use FILE * instead of a file descriptor.  This will make it a bit
easier/more consistent when we implement support for the -c option.

Makefile

@@ -3,13 +3,14 @@ include config.mk
 .POSIX:
 .SUFFIXES: .c .o
 
-HDR = fs.h text.h util.h arg.h
+HDR = crypt.h fs.h text.h md5.h sha1.h util.h arg.h
 LIB = \
 	util/afgets.o    \
 	util/agetcwd.o   \
 	util/apathmax.o  \
 	util/concat.o    \
 	util/cp.o        \
+	util/crypt.o     \
 	util/enmasse.o   \
 	util/eprintf.o   \
 	util/estrtol.o   \