regcomp.c: Revert mistakenly changed line

Commit 31f05a37 mistakenly included a line that is supposed to be in a
future commit.  This reverts that until then; and adds a test for it, as
this was not caught by the test suite.
pp.c: Silence compiler warning

The only time the result of toFOLD_LC() can be larger than a byte is
in a UTF-8 locale, which has already been ruled out for this section of
code.
handy.h: Add a cBOOL()

isascii() is used as a fallback for isASCII().  This would be on an
unusual platform or under unusual circumstances.  isascii() may return
values besides 0 and 1 which can cause things that are expecting a bool
to fail.