lxc-ls: check for ENOMEM and tweaking

lxc-ls: check for ENOMEM and tweaking

- If lxc_container_new() fails we check for ENOMEM and if so goto out. If
  ENOMEM is not set we will simply continue. The same goes for the call to
  regcomp() but instead of checking for ENOMEM we need to check for REG_ESPACE.

- Tweaking: Since lxc-ls might have to gather a lot of containers and I don't
  know if compilers will always optimize this, let's move *some* variable
  declarations outside of the loop when it does not hinder readability.

- Set ls_nesting to 0 initially. Otherwise users will always see nested
  containers printed.

- ls_get() gains an argument char **lockpath which is a string pointing us to
  the lock we put under /run/lxc/lock/.../... so that we can remove the lock
  when we no longer need it. To avoid pointless memory allocation in each new
  recursion level, we share lockpath amongst all non-fork()ing recursive calls
  to ls_get().  As it is not guaranteed that realloc() does not do any memory
  moving when newlen == len_lockpath, we give ls_get() an additional argument
  size_t len_lockpath). Every time we have a non-fork()ing recursive call to
  ls_get() we check if newlen > len_lockpath and only then do we
  realloc(*lockpath, newlen * 2) a reasonable chunk of memory (as the path will
  keep growing) and set len_lockpath = newlen * 2 to pass to the next
  non-fork()ing recursive call to ls_get().
  To avoid keeping a variable char *lockpath in main() which serves no purpose
  whatsoever and might be abused later we use a compound literal
  &(char *){NULL} which gives us an anonymous pointer. This pointer we can use
  for memory allocation in ls_get() for lockpath. We can conveniently free() it
  in ls_get() when the nesting level parameter lvl == 0 after exiting the loop.
  The advantage is that the variable is only accessible within ls_get() and not
  in main() while at the same time giving us an easy way to share lockpath
  amongst all non-fork()ing recursive calls to ls_get().

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>