Fix genstr/genconf option in asn1parse

Fix genstr/genconf option in asn1parse

At some point the asn1parse applet was changed to default the inform to
PEM, and defalt input file to stdin.  Doing so broke the -genstr|conf options,
in that, before we attempt to generate an ASN1 block from the provided
genstr string, we attempt to read a PEM input from stdin.  As a result,
this command:
openssl asn1parse -genstr OID:1.2.3.4
hangs because we are attempting a blocking read on stdin, waiting for
data that never arrives

Fix it by giving priority to genstr|genconf, such that, if set, will just run
do_generate on that string and exit

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22957)