Fixes openssl#5778, openssl#5840

The various IS_*() macros did not work correctly for 8-bit ASCII
characters with the high bit set, because the CVT(a) preprocessor
macro and'ed the given ASCII value with 0x7F, effectively folding
the high value range 128-255 over the low value range 0-127.
As a consequence, some of the IS_*() erroneously returned TRUE.

This commit fixes the issue by mapping CVT(a) to 127 for all values
a >= 127. This works because the lookup tables CONF_type_default
and CONF_type_win32 have all bits cleared at entry 127, whence
IS_*(a) returns FALSE for all values outside the range 0-127.

The IS_*() macros were also changed to return TRUE or FALSE (1 or 0)
instead of a nonzero or zero value.

Note that with this change, the macro IS_*(c,a) evaluates the 'a'
argument twice, unless CHARSET_EBCDIC is defined. This prohibits the
use of arguments with side effects like IS_*(c, *p++).

...to get rid of double argument evaluation in the CVT() macro.