[mod_openssl] read secret data w/ BIO_new_mem_buf
[mod_openssl] read secret data w/ BIO_new_mem_buf

Using BIO_new_mem_buf() allows lighttpd to control file reads and
securely wiping memory, avoiding the use of BIO_s_file(), which
employs stdio and BIO internal copying without securely wiping memory.

BIO_new_mem_buf() is also much more performant than BIO_s_file()
or BIO_new_fd() without a buffering BIO, which might make many
syscalls reading a single character at a time.

BIO_new_mem_buf() appears to be supported by older OpenSSL versions
as well as by numerous OpenSSL-compatible APIs, e.g. in BoringSSL,
LibreSSL, WolfSSL

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4