[3769] Added support for creating PIDFiles to dhcpsrv::Daemon

[3769] Added support for creating PIDFiles to dhcpsrv::Daemon

src/lib/dhcpsrv/daemon.h/cc
    New methods:
    static void setConfigFile(const std::string& config_file);
    std::string getProcName() const;
    setProcName(const std::string& proc_name);
    std::string getPIDFileDir() const;
    void setPIDFileDir(const std::string& pid_file_dir);
    std::string getPIDFileName() const;
    void setPIDFileName(const std::string& pid_file_name);
    void createPIDFile(int pid = 0);
    std::string makePIDFileName() const;

    New members:
    std::string proc_name_;
    std::string pid_file_dir_;
    isc::util::PIDFilePtr pid_file_;

src/lib/dhcpsrv/tests/daemon_unittest.cc
    New tests:
    TEST_F(DaemonTest, getSetConfigFile)
    TEST_F(DaemonTest, getSetProcName)
    TEST_F(DaemonTest, getSetPIDFileDir)
    TEST_F(DaemonTest, setPIDFileName)
    TEST_F(DaemonTest, makePIDFileName)
    TEST_F(DaemonTest, createPIDFile)
    TEST_F(DaemonTest, createPIDFileOverwrite)
    TEST_F(DaemonTest, PIDFileCleanup)

src/lib/util/pid_file.h
    Added typedef boost::shared_ptr<PIDFile> PIDFilePtr;