doc: Add manual page generation for bitcoin-util

- Add `-version` option to `bitcoin-util`
- Add `bitcoin-util` call to `gen-manpages.sh`
- Add stub manual page `bitcoin-util.1`
- Add install of `bitcoin-util.1` to build system

doc/man/bitcoin-util.1

@@ -0,0 +1,5 @@
+.TH BITCOIN-UTIL "1"
+.SH NAME
+bitcoin-util \- manual page for bitcoin-util
+
+This is a placefolder file. Please follow the instructions in \fIcontrib/devtools/README.md\fR to generate the manual pages after a release.