syscalls: Add 'swapon' and 'swapoff'.

* guix/build/syscalls.scm (swapon, swapoff): New procedures.
* tests/syscalls.scm ("swapon, ENOENT/EPERM", "swapoff, EINVAL/EPERM"):
  New tests.

syscalls: Add 'all-network-interfaces'.

* guix/build/syscalls.scm (network-interfaces): Update docstring.
  (%interface-line): New variable.
  (all-network-interfaces): New procedure.
* tests/syscalls.scm ("all-network-interfaces"): New test.
  ("network-interfaces"): Change to make sure the result is a subset
  of (all-network-interfaces).

syscalls: Add 'network-interface-flags'.

* guix/build/syscalls.scm (SIOCGIFFLAGS, IFF_UP, IFF_BROADCAST,
  IFF_LOOPBACK, IF_NAMESIZE): New variables.
  (network-interface-flags, loopback-network-interface?): New
  procedures.
* tests/syscalls.scm ("network-interface-flags",
  "loopback-network-interface?"): New tests.

syscalls: Add 'network-interfaces'.

* guix/build/syscalls.scm (SIOCGIFCONF, ifconf-struct,
  ifreq-struct-size): New variables.
  (%ioctl, bytevector->string-list, network-interfaces): New
  procedures.
* tests/syscalls.scm ("network-interfaces"): New test.

Merge branch 'master' into core-updates

syscalls: Be more permissive in 'umount' test.

* tests/syscalls.scm ("umount, ENOENT"): Rename to...
  ("umount, ENOENT/EPERM"): ... this.  Accept EPERM as a valid return
  value.

Add (guix build syscalls).

* guix/build/syscalls.scm, tests/syscalls.scm: New files.
* Makefile.am (MODULES): Add guix/build/syscalls.scm.
  (SCM_TESTS): Add tests/syscalls.scm.
* guix/utils.scm (%libc-errno-pointer, errno): Remove; take from (guix
  build syscalls).