Log Message:
fehqlibs: update to 25a. Changes:

- Removed 'register' in functions. Added 'const' for input strings.
- Compatibility with current gcc and clang compilers:
- (Internal) Unrolling for() loops removed. Internal functions are
  static now.
- (Internal) buffer_init uses now 'buffer_unixread' and
  'buffer_unixwrite' with matching signatures.
- (External) Added getopb() as external and thus removing getop()
  compatibility.
- (Installation) Moved C source files to ./src. Added INSTALL.md to
  describe changes.
- (Compilation) Added -fwrapv flag to the compiler (correctly handling
  two's complement).
- (Internal) alloc_re()'s first argument is now a void pointer to a list
  [gcc 14.2].