keychain: fixup shebang, use upstream builder, install manpage
keychain: fixup shebang, use upstream builder, install manpage

fixup shebang - build was not running fixupPhase, and shebang was pointing to
/bin/sh. On a system where /bin/sh links to dash, this results in errors:

    /nix/store/v40yk16fl1rhvr6zz91ij753ziyphwwp-keychain-2.8.0/bin/.keychain-wrapped: 59: /nix/store/v40yk16fl1rhvr6zz91ij753ziyphwwp-keychain-2.8.0/bin/.keychain-wrapped: shopt: not found
    /nix/store/v40yk16fl1rhvr6zz91ij753ziyphwwp-keychain-2.8.0/bin/.keychain-wrapped: 686: /nix/store/v40yk16fl1rhvr6zz91ij753ziyphwwp-keychain-2.8.0/bin/.keychain-wrapped: Syntax error: "(" unexpected (expecting ")")

use upstream builder - the include Makefile uses perl to generate a manpage and
insert the same text into the keychain script for '--help' output, which was
otherwise missing:

    $ keychain --help

     * keychain 2.8.0 ~ http://www.funtoo.org

    [snip]

    INSERT_POD_OUTPUT_HERE

install manpage - self explanatory

SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,SymbiFlow/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,triton/triton,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,triton/triton,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,triton/triton,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,triton/triton,NixOS/nixpkgs