Break out code from Perl_sv_gets() which is self contained and exits directly.

Break out code from Perl_sv_gets() which is self contained and exits directly.

Previously the code for handling fixed length records, and appending from a
non-UTF-8 handle to a UTF-8 scalar, was handled in two blocks in Perl_sv_gets()
which exited via a goto to the last statement of Perl_sv_gets(). By breaking
them out into separate static functions, the control flow becomes clearer.