maint: update quoting of "Try `prog --help'" to "Try 'prog --help'"

maint: update quoting of "Try `prog --help'" to "Try 'prog --help'"

Automatically adjust both the source (now in only one place)
and all tests that expect the resulting output via this:

git grep -l 'Try.*--help' src/system.h tests \
  | xargs perl -pi -e 's/Try \\?`(\S+ --help)/Try '\''$1/'