Fix build issues caused by -Wsign-compare and -Wincompatible-pointer-…

[lsandov1]

…types

In file included from ../../grub-core/net/http.c:19:
../../grub-core/net/http.c: In function ‘http_establish’:
../../grub-core/net/http.c:417:48: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘enum <anonymous>’ due to unsignedness of other operand [-Wsign-compare]
  417 |                 data->filename, server, port ? port : HTTP_PORT);
      |                                                ^~~~

../../grub-core/net/bootp.c: In function ‘grub_dhcp6_session_add’:
../../grub-core/net/bootp.c:1061:46: error: passing argument 2 of ‘grub_datetime2unixtime’ from incompatible pointer type [-Wincompatible-pointer-types]
 1061 |   if (err || !grub_datetime2unixtime (&date, &t))
      |                                              ^~
      |                                              |
      |                                              grub_int32_t * {aka int *}

In file included from ../../grub-core/net/bootp.c:27:
../../include/grub/datetime.h:55:77: note: expected ‘grub_int64_t *’ {aka ‘long int *’} but argument is of type ‘grub_int32_t *’ {aka ‘int *’}
   55 | grub_datetime2unixtime (const struct grub_datetime *datetime, grub_int64_t *nix)
      |                                                               ~~~~~~~~~~~~~~^~~

../../grub-core/net/drivers/efi/efinet.c: In function ‘grub_efi_net_config_real’:
../../grub-core/net/drivers/efi/efinet.c:854:50: error: initialization of ‘struct grub_net_bootp_packet *’ from incompatible pointer type ‘grub_efi_pxe_packet_t *’ [-Wincompatible-pointer-types]
  854 |         struct grub_net_bootp_packet *dhcp_ack = &pxe_mode->dhcp_ack;
      |                                                  ^

../../grub-core/net/drivers/efi/efinet.c:859:57: error: initialization of ‘struct grub_net_bootp_packet *’ from incompatible pointer type ‘grub_efi_pxe_packet_t *’ [-Wincompatible-pointer-types]
  859 |             struct grub_net_bootp_packet *proxy_offer = &pxe_mode->proxy_offer;
      |                                                         ^

../../grub-core/net/drivers/efi/efinet.c: In function ‘grub_efi_net_config_real’:
../../grub-core/net/drivers/efi/efinet.c:859:57: error: initialization of ‘struct grub_net_bootp_packet *’ from incompatible pointer type ‘grub_efi_pxe_packet_t *’ [-Wincompatible-pointer-types]
  859 |             struct grub_net_bootp_packet *proxy_offer = &pxe_mode->proxy_offer;
      |                                                         ^

[lsandov1]

These and other related changes found by the static analyzer will be collected at some point and create a larger MR