libre2c_posix: extended regex_t structure to hold more submatch data.

libre2c_posix: extended regex_t structure to hold more submatch data.

Added fields:
    - re_nsub: total number of submatch groups, required by POSIX standard

    - pmatch: buffer for submatch results, usually supplied by the user to
      regexec(), but we allow to do the allocation and storage in regex_t.
      This is convenient for users that have hard time managing memory, e.g.
      java bindings to libre2c_posix.

    - regs: buffer for internal use by regexec(), strored in regex_t to
      avoid repeated memory allocation on each call to regexec() with the
      same regex.