Update MD5 cryptography call usage.

[fangfufu]

Right now when httpdirfs is compiled, I get the following error messages:

src/util.c: In function ‘generate_md5sum’:
src/util.c:122:5: warning: ‘MD5_Init’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
  122 |     MD5_Init(&c);
      |     ^~~~~~~~
In file included from src/util.c:6:
/usr/include/openssl/md5.h:49:27: note: declared here
   49 | OSSL_DEPRECATEDIN_3_0 int MD5_Init(MD5_CTX *c);
      |                           ^~~~~~~~
src/util.c:123:5: warning: ‘MD5_Update’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
  123 |     MD5_Update(&c, str, len);
      |     ^~~~~~~~~~
/usr/include/openssl/md5.h:50:27: note: declared here
   50 | OSSL_DEPRECATEDIN_3_0 int MD5_Update(MD5_CTX *c, const void *data, size_t len);
      |                           ^~~~~~~~~~
src/util.c:124:5: warning: ‘MD5_Final’ is deprecated: Since OpenSSL 3.0 [-Wdeprecated-declarations]
  124 |     MD5_Final(md5, &c);
      |     ^~~~~~~~~
/usr/include/openssl/md5.h:51:27: note: declared here
   51 | OSSL_DEPRECATEDIN_3_0 int MD5_Final(unsigned char *md, MD5_CTX *c);
      |                           ^~~~~~~~~

These API usages need to be updated.

[fangfufu]

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