Fix a bunch of conversion warnings #197

This leaves the following warnings:

    ../src/ZNCString.cpp: In member function 'CString& CString::MakeUpper()':
    ../src/ZNCString.cpp:128:16: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'CString& CString::MakeLower()':
    ../src/ZNCString.cpp:137:16: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'CString CString::AsUpper() const':
    ../src/ZNCString.cpp:148:16: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'CString CString::AsLower() const':
    ../src/ZNCString.cpp:159:16: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'CString CString::Escape_n(CString::EEscape, CString::EEscape) const':
    ../src/ZNCString.cpp:207:84: warning: conversion to 'unsigned int' from 'long int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:210:13: warning: conversion to 'unsigned char' from 'unsigned int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:239:26: warning: conversion to 'unsigned char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:241:39: warning: conversion to 'unsigned char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:246:22: warning: conversion to 'unsigned char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:248:35: warning: conversion to 'unsigned char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'long unsigned int CString::Base64Decode(CString&) const':
    ../src/ZNCString.cpp:820:37: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:827:47: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp:834:70: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'short int CString::ToShort() const':
    ../src/ZNCString.cpp:978:81: warning: conversion to 'short int' from 'long unsigned int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'short unsigned int CString::ToUShort() const':
    ../src/ZNCString.cpp:979:91: warning: conversion to 'short unsigned int' from 'long unsigned int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'unsigned int CString::ToUInt() const':
    ../src/ZNCString.cpp:980:87: warning: conversion to 'unsigned int' from 'long unsigned int' may alter its value [-Wconversion]
    ../src/ZNCString.cpp: In member function 'int CString::ToInt() const':
    ../src/ZNCString.cpp:981:77: warning: conversion to 'int' from 'long unsigned int' may alter its value [-Wconversion]
    ../src/main.cpp: In function 'void seedPRNG()':
    ../src/main.cpp:111:35: warning: conversion to 'unsigned int' from '__suseconds_t {aka long int}' may alter its value [-Wconversion]
    ../src/FileUtils.cpp: In member function 'ssize_t CFile::Write(const char*, size_t)':
    ../src/FileUtils.cpp:415:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    ../../modules/dcc.cpp: In member function 'virtual void CDCCMod::OnModCTCP(const CString&)':
    ../../modules/dcc.cpp:226:33: warning: conversion to 'unsigned int' from 'long unsigned int' may alter its value [-Wconversion]
    ../../modules/dcc.cpp: In member function 'virtual void CDCCSock::ReadData(const char*, size_t)':
    ../../modules/dcc.cpp:312:40: warning: conversion to 'uint32_t {aka unsigned int}' from 'long unsigned int' may alter its value [-Wconversion]
    ../../modules/q.cpp: In member function 'CString CQModule::HMAC_MD5(const CString&, const CString&)':
    ../../modules/q.cpp:447:21: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../../modules/q.cpp:448:21: warning: conversion to 'char' from 'int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp: In member function 'bool CSASLMod::AuthenticateBlowfish(const CString&)':
    ../../modules/sasl.cpp:225:69: warning: conversion to 'uint16_t {aka short unsigned int}' from 'unsigned int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp:226:36: warning: conversion to 'uint16_t {aka short unsigned int}' from 'unsigned int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp:246:56: warning: conversion to 'uint16_t {aka short unsigned int}' from 'unsigned int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp:247:36: warning: conversion to 'uint16_t {aka short unsigned int}' from 'unsigned int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp:261:56: warning: conversion to 'uint16_t {aka short unsigned int}' from 'unsigned int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp:262:36: warning: conversion to 'uint16_t {aka short unsigned int}' from 'unsigned int' may alter its value [-Wconversion]
    ../../modules/sasl.cpp:318:63: warning: conversion to 'uint16_t {aka short unsigned int}' from 'int' may alter its value [-Wconversion]