Fix DER reading from stdin for BIO_f_readbuffer

Fixes #14559

The intitial implementation of the gets() function tried using the next bio's gets() function.
For a file BIO this r

Fix DER reading from stdin for BIO_f_readbuffer

Fixes #14559

The intitial implementation of the gets() function tried using the next bio's gets() function.
For a file BIO this returned incorrect data for binary data containing 0x00.
Just buffering all data during gets() did not work however since some
applications open and close the bio multiple times when dealing with pem
files containing multiple entries.. This does not work
when reading from stdin unless the data if buffered one byte at a time.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14599)

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Add new filter BIO BIO_f_readbuffer()

This allows BIO_tell() and BIO_seek() to work for BIO's that do
not support these methods. The main use case for this is file/fd BIO's
that use

Add new filter BIO BIO_f_readbuffer()

This allows BIO_tell() and BIO_seek() to work for BIO's that do
not support these methods. The main use case for this is file/fd BIO's
that use stdin.

This works for stdin taken from input redirection (command < file),
and stdin via pipe (cat file | command).

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14407)

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