Until now, the "http" device only worked when firmware itself booted
loader.efi via EFI HTTP Boot: efihttp_dev_init() required a URI node on
the loader's own boot device path, and every fetch went to that same
server. Add a second form, using the parse_uri() helper from libsa/dev.c,
that works anywhere: http[N]://host[:port]/path (N is a network interface
unit, defaulting to 0). This lets a devspec like this be used as rootdev,
in loader.conf, or interactively, independent of how loader.efi itself
was booted.

Network configuration is shared, loader-wide state (the myip/netmask/
gateip/nameip globals from net.h): if something has already configured
them (e.g. tftp/nfs via net_open()), the http device just reuses those
values, applying them to EFI_IP4_CONFIG2_PROTOCOL as a static address. If
nothing has, it drives IP4Config2's own DHCP directly instead of calling
netdev's net_open(): net_open() opens the NIC's SNP
EFI_OPEN_PROTOCOL_EXCLUSIVE, which would disconnect the Mnp/Ip4/.../
HttpDxe driver chain this code depends on. That chain is also commonly
disconnected already by whatever chained us here (e.g. iPXE excludes and
exclusively opens SNP for its own raw I/O), so reconnect it explicitly
with ConnectController() before giving up.

The legacy EFI HTTP Boot path is untouched.