Bug 4843 pt3: GCC-8 fixes and refactoring (#172)

Bug 4843 pt3: GCC-8 fixes and refactoring (#172)

GCC-8 enables a lot more warnings related to unsafe coding
practices. The old Squid code contains a lot of risky buffer
size assumptions and implicit assumptions about C-string strcat,
strncat and snprintf  changes when operating on those buffers -
many can result in output truncation. Squid's use of -Werror
makes these many issues all go from warnings to outright
compile failures.

Rather than just extending the char* buffer sizes not to
truncate this work seeks to actually remove the issues
permanently by converting to SBuf and updated Squid coding
styles.

The C++1z compilers (GCC-8 and Clang 4.0) are beginning to warn
about C functions memset/memcpy/memmove being used on class
objects which lack "trivial copy" constructor or assignment
operator - their use is potentially unsafe where anything more
complex than trivial copy/blit is required. A number of classes
in Squid are safely copied or initialized with those functions
for now but again the -Werror makes these hard errors.
Completing affected objects conversion from C to C++ code avoids
any deeply hidden issues or adding compiler exceptions to
silence the warnings.

see individual commit messages for details on the particular
changes each does.