Where possible, change gv_fetchfile() to gv_fetchfile_flags(),

Where possible, change gv_fetchfile() to gv_fetchfile_flags(),
gv_stashpv() to gv_stashpvn() and gv_fetchpv() to gv_fetchpvn_flags().
Change the len parameter of S_find_in_my_stash() from I32 to STRLEN, as
a pointer the variable needs to be passed onwards, and size matters on
64 bit platforms.
Fix the temporary scribbling of a buffer in Perl_yylex() by using
gv_fetchpvn_flags(), and remove the XXX comment added in change 27641.
Brought to you by the Campaign for the Elimination of strlen().

p4raw-id: //depot/perl@32127