Misc improvements with minor breakage of includes

1. [Change] Move HRTN() & OXCPT() from LogStream.h to StrUtil.h.
2. [Change] Rewrite I/O manipulator bux::timestamp utilizing fmt::format() to shorten the boilerplate code.
3. [Change] Quit assuming std::string::begin() is defined as const char * upon implementing ctor of bux::C_IMemBuf.
4. [Change] Harden the criteria to make flag values prefixed with '-' (negative numbers) hardly missed (i.e. misunderstood as short-named flags).
5. [Change] Claim that "Behavior of adding duplicate subcommands or flags of the same name is undefined." i.e it is caller's responsibility.
6. [Change] No more prepend ./ to the command name of USAGE part of help text.
7. [Change] Allow bux::scanFile() to provide encoding code of the input stream. The change of function prototype has a virtue of preventing short strings from guessing the wrong encoding.
8. [Bug] C_ScannerImpl<>::add() may throw due to losing its track of the last non-null action function pointer.
9. [Change] Were C_ScannerImpl<>::add() thrown, the incorporated message will have value of m_CurState.