Doc: replace `NULL` terminated with `NUL`

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16885)

doc/internal/man3/ossl_punycode_decode.pod

@@ -22,10 +22,10 @@ PUNYCODE encoding introduced in RFCs 3490-3492 is widely used for
 representation of host names in ASCII-only format. Some specifications,
 such as RFC 8398, require comparison of host names encoded in UTF-8 charset.
 
-ossl_a2ulabel() decodes NULL-terminated hostname from PUNYCODE to UTF-8,
+ossl_a2ulabel() decodes NUL-terminated hostname from PUNYCODE to UTF-8,
 using a provided buffer for output.
 
-ossl_a2ucompare() accepts two NULL-terminated hostnames, decodes the 1st
+ossl_a2ucompare() accepts two NUL-terminated hostnames, decodes the 1st
 from PUNYCODE to UTF-8 and compares it with the 2nd one as is.
 
 ossl_punycode_decode() decodes one label (one dot-separated part) from

doc/man3/BN_bn2bin.pod

@@ -94,7 +94,7 @@ BN_bin2bn() returns the B<BIGNUM>, NULL on error.
 BN_bn2binpad() returns the number of bytes written or -1 if the supplied
 buffer is too small.
 
-BN_bn2hex() and BN_bn2dec() return a null-terminated string, or NULL
+BN_bn2hex() and BN_bn2dec() return a NUL-terminated string, or NULL
 on error. BN_hex2bn() and BN_dec2bn() return the number of characters
 used in parsing, or 0 on error, in which
 case no new B<BIGNUM> will be created.

doc/man3/SCT_print.pod

@@ -31,7 +31,7 @@ beforehand in order to set the validation status of an SCT first.
 
 =head1 RETURN VALUES
 
-SCT_validation_status_string() returns a null-terminated string representing
+SCT_validation_status_string() returns a NUL-terminated string representing
 the validation status of an B<SCT> object.
 
 =head1 SEE ALSO

doc/man3/SSL_CIPHER_get_name.pod

@@ -157,7 +157,7 @@ Some examples for the output of SSL_CIPHER_description():
 
 SSL_CIPHER_get_name(), SSL_CIPHER_standard_name(), OPENSSL_cipher_name(),
 SSL_CIPHER_get_version() and SSL_CIPHER_description() return the corresponding
-value in a null-terminated string for a specific cipher or "(NONE)"
+value in a NUL-terminated string for a specific cipher or "(NONE)"
 if the cipher is not found.
 
 SSL_CIPHER_get_bits() returns a positive integer representing the number of

doc/man3/SSL_CTX_set_keylog_callback.pod

@@ -29,7 +29,7 @@ The key logging callback is called with two items: the B<ssl> object associated
 with the connection, and B<line>, a string containing the key material in the
 format used by NSS for its B<SSLKEYLOGFILE> debugging output. To recreate that
 file, the key logging callback should log B<line>, followed by a newline.
-B<line> will always be a NULL-terminated string.
+B<line> will always be a NUL-terminated string.
 
 =head1 RETURN VALUES
 

doc/man3/SSL_group_to_name.pod

@@ -20,7 +20,7 @@ or SSL_get_shared_group().
 =head1 RETURN VALUES
 
 If non-NULL, SSL_group_to_name() returns the TLS group name
-corresponding to the given I<id> as a NULL-terminated string.
+corresponding to the given I<id> as a NUL-terminated string.
 If SSL_group_to_name() returns NULL, an error occurred; possibly no
 corresponding tlsname was registered during provider initialisation.