mark Perl_my_strftime with format attribute

mark this function with

    __attribute__format__null_ok__(__strftime__,pTHX_1,0)

so that compiler checks and warnings about strftime-style format args
can be checked.

Rather than adding new flag(s) to embed.fnc, I just enhanced the f flag
to treat it as strftime-style rather than printf if the function name
matches /strftime/. This was quicker, and we're unlikely to have many
such functions.
add GCC_DIAG_IGNORE(), GCC_DIAG_RESTORE macros

These allow you to temporarily disable a specific gcc or clang warning;
e.g.

    GCC_DIAG_IGNORE(-Wmultichar);
    char b = 'ab';
    GCC_DIAG_RESTORE;
silence -Wformat-nonliteral compiler warnings

Due to the security risks associated with user-supplied formats
being passed to C-level printf() style functions (eg %n),
gcc has a -Wformat-nonliteral warning that complains whenever such a
function is passed a non-literal format string.

This commit silences all such warnings in core and ext/.

The main changes are

1) the 'f' (format) flag in embed.fnc is now handled slightly more
cleverly. Rather than just applying to functions whose last arg is '...'
(and where the format arg is assumed to be the previous arg), it
can now handle non-'...' functions: arg checking is disabled, but format
checking is sill done: it works by assuming that an arg called 'fmt',
'pat' or 'f' is the format string (and dies if fails to find exactly one
such arg).

2) with the new embed.fnc functionally, more functions have been marked
with the 'f' flag. When such a function passes its fmt arg onto an inner
printf-like function, we simply disable the warning for that call using
GCC_DIAG_IGNORE(-Wformat-nonliteral), since we know that the caller must
have already checked it.

3) In quite a few places the format string isn't literal, but it *is*
constant (e.g. PL_warn_uninit_sv). For those cases, again disable the
warning.

4) In pp_formline(), a particular format was was one of several different
literal strings depending on circumstances. Rather than assigning this
string to a temporary variable, incorporate the ?: branches directly in
the function call arg. gcc is clever enough to decide the arg is then
always literal.