PR 81552 Improve and document -flag-init-integer

PR 81552 Improve and document -flag-init-integer

Make the option handling code parse the -flag-init-integer value as a
C long type, allowing a larger range on systems where long is a larger
type than int.  Document the behavior.

Regtested on x86_64-pc-linux-gnu, committed as obvious.

2019-02-14  Janne Blomqvist  <jb@gcc.gnu.org>

        PR fortran/81552
        * gfortran.h (gfc_option_t): Make flag_init_integer_value a long.
        * options.c (gfc_handle_option): Use strtol instead of atoi.
        * invoke.texi: Document -finit-integer behavior in more detail

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@268906 138bc75d-0d04-0410-961f-82ee72b054a4