common: Add null byte to serialized std::string

This change makes network packets using std::string compatible with
messages like CLTOMA_FUSE_SYMLINK or MATOCL_FUSE_READLINK, which use
null-terminated strings with 32-bit length.

Change-Id: Ia21e44a6987090eec95d8280ad7ada09e7ca74f4
utils: Add new string types to wireshark plugin

This commit adds a possibility to use STRING8 (aka. NAME) and
STRING32 (aka. STDSTRING) field types when describing a network
packet. These are two types of strings used in network
communication between LizardFS modules. The format is as follows:
* STRING8: len:8 string:STRING[len]
* STRING32: len:32 string:STRING[len]

Change-Id: I6fbe4963018b1d1bdcf3d0561d875ee278549920
master, mount: Fix checking permissions during 'touch'

man utime says:

  The utime() system call changes the access and modification times of the
  inode specified by filename to the actime and modtime fields of times
  respectively.
  If times is NULL, then the access and modification times of the file are
  set to the current time.
  Changing timestamps is permitted when: either the process has appropriate
  privileges, or the effective user ID equals the user ID of the file, or
  times is NULL and the process has write permission for the file.

The master server couldn't distinguish between the NULL and non-NULL
version, so it was always checking for the ownership of the changed
file. This commit reuses two no longer used setattr flags to pass the
required information to the master server and modifies the master server
to check permissions correctly.

Change-Id: I63ff90e6714a7383b016f2d54b0ecf848bc15424