[arch-releng] [RFCv2] Add PXE HTTP/FTP support with curl

[Gerardo Exequiel Pozzi]

In this second version also allow to download only needed *.sfs files
from unpacked ISO image, instead of the whole ISO.

For example running an small http daemon (darkhttp) inside live enviroment:
(plus tftpd and dhcpd)

[root at archiso ~]# darkhttpd /run/archiso/bootmnt/
darkhttpd/1.8, copyright (c) 2003-2011 Emil Mikulic.
listening on: http://0.0.0.0:80/

Then booting from PXE in another machine with boot parameter
archiso_curl_url=http://${pxeserver}/ results in:

:: Running Hook [archiso_pxe_common]
IP-Config: eth0 hardware address 52:54:00:12:34:57 mtu 1500
IP-Config: eth0 guessed broadcast address 192.168.0.255
IP-Config: eth0 guessed nameserver address 192.168.0.218
IP-Config: eth0 complete (from 192.168.0.218):
 address: 192.168.0.90     broadcast: 192.168.0.255    netmask: 255.255.255.0
 gateway: 192.168.0.218    dns0     : 192.168.0.218    dns1   : 0.0.0.0
 rootserver: 192.168.0.218 rootpath:
 filename  :
:: Running Hook [archiso_pxe_nbd]
:: Running Hook [archiso_pxe_curl]
:: Running Hook [archiso_pxe_nfs]
:: Mounting /run/archiso/curlspace (tmpfs) filesystem, size='75%'
:: Downloading 'http://192.168.0.218/arch/aitab'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   298  100   298    0     0  15051      0 --:--:-- --:--:-- --:--:-- 17529
:: Downloading 'http://192.168.0.218/arch/i686/root-image.fs.sfs'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 84.5M  100 84.5M    0     0   106M      0 --:--:-- --:--:-- --:--:--  107M
:: Downloading 'http://192.168.0.218/arch/i686/lib-modules.fs.sfs'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 33.7M  100 33.7M    0     0  97.9M      0 --:--:-- --:--:-- --:--:-- 98.3M
:: Downloading 'http://192.168.0.218/arch/any/usr-share.fs.sfs'
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 52.6M  100 52.6M    0     0   100M      0 --:--:-- --:--:-- --:--:--  100M
:: Mounting /run/archiso/cowspace (tmpfs) filesystem, size=75%...