Fix -h/-i/-v/-t single-char flags being silently hijacked by user function arguments

[anishesg]

When a user function has a parameter whose name starts with h, i, v, or t
(e.g. headless, interactive, verbose), Fire's shortcut-flag expansion in
_ParseKeywordArgs (fire/core.py) would match a bare -h/-i/-v/-t flag
to that parameter before Fire's own help/interactive/verbose/trace flags had a
chance to handle it. This caused python myfile.py -h to silently set
headless=True instead of showing the help screen.

The root cause is that the single-character shortcut expansion in
_ParseKeywordArgs (around the elif len(key) == 1 branch) had no awareness
of Fire's own reserved single-character flags defined in parser.CreateParser.

The fix introduces a module-level frozenset _FIRE_RESERVED_SINGLE_CHAR_FLAGS
containing {'h', 'i', 'v', 't'} and skips shortcut expansion for those
characters, letting them fall through to remaining_kwargs where
_IsHelpShortcut (and the normal flag-arg parsing path) can process them
correctly. A regression test covering the headless/-h collision case is
included in fire/core_test.py.

Fixes #238