Fix myesc() 'sptr' conditionals

Fix myesc() 'sptr' conditionals

* Don't call isascii() here. It's deprecated in POSIX and not needed
  for myesc's case.
* The check of the character class and range here should match what's
  defined as {ESCSEQ} in scan.l, so for [[:xdigit:]] we use isxdigit();
  for [0-7] we check '0' <= c <= '7' (not isdigit(c) because isdigit is
  locale-dependant in standard's sense)
* Add missing length limit for "\x<hex>" (<hex> is at most 2 digits)