BUG/MINOR: tools: also accept '+' as a valid character in an identifier

BUG/MINOR: tools: also accept '+' as a valid character in an identifier

The function is_idchar() was added by commit 36f586b ("MINOR: tools:
add is_idchar() to tell if a char may belong to an identifier") to
ease matching of sample fetch/converter names. But it lacked support
for the '+' character used in "base32+src" and "url32+src". A quick
way to figure the list of supported sample fetch+converter names is
to issue the following command:

   git grep '"[^"]*",.*SMP_T_.*SMP_USE_'|cut -f2 -d'"'|sort -u

No more entry is reported once searching for characters not covered
by is_idchar().

No backport is needed.