chmod: Disallow negative mode, document mode affected by umask.

mkdir-mode: Disallow negative mode, document that filesystems
may interpret the mode in different ways.

mknod: Disallow negative mode, document mode affected by umask.

umask: Check the range of umask mask value carefully.
(cherry picked from commit 61ab83d19009a8006dd73ebe16d22494b78be4d1)

The RPC stubs already prefix the command name to error messages.
The daemon doesn't have to do this.  As a (small) benefit this also
makes the daemon slightly smaller.

Code in the daemon such as:

  if (argv[0] == NULL) {
    reply_with_error ("passed an empty list");
    return NULL;
  }

now results in error messages like this:

  ><fs> command ""
  libguestfs: error: command: passed an empty list

(whereas previously you would have seen ..command: command:..)

git grep -l 'strcmp *([^=]*== *0'|xargs \
  perl -pi -e 's/\bstrcmp( *\(.*?\)) *== *0/STREQ$1/g'

truncate, truncate_size: Used to truncate files to a particular
size, or to zero bytes.

mkdir_mode: Like mkdir but allows you to also specify the
initial permissions for the new directory.

utimens: Set timestamp on a file with nanosecond accuracy.

lchown: Corresponding to lchown(2) syscall (we already have chown).

The implementation is complicated by the fact that we had to
add an Int64 parameter type to the generator.

* daemon/dir.c (do_mkdtemp): Rewrite for a "const" parameter.
* src/generator.ml (mkdtemp): Declare parameter to be of type Pathname.

Nearly every file-related function in daemons/*.c is affected:
Remove this pair of statements from each affected do_* function:
-  NEED_ROOT (return -1);
-  ABS_PATH (dir, return -1);
and change the type of the corresponding parameter to "const char *".
* src/generator.ml: Emit NEED_ROOT just once, even when there are two or
more Pathname args.

run this command:
  git grep -l -w NEED_ROOT|xargs perl -pi -e \
    's/(NEED_ROOT) \((.*?)\)/$1 (return $2)/'

run this command:
  git grep -l -w ABS_PATH|xargs perl -pi -e \
    's/(?:ABS_PATH)( \(.*?,) (.*?)\)/ABS_PATH$1 return $2)/'

Do it by running this command:
[exempted files are matched via .x-sc_TAB_in_indentation]

  git ls-files \
    | pcregrep -vf .x-sc_TAB_in_indentation \
    | xargs pcregrep -l '^ *\t' \
    | xargs perl -MText::Tabs -ni -le \
      '$m=/^( *\t[ \t]*)(.*)/; print $m ? expand($1) . $2 : $_'