vdagent: add as_user to run tasks with user privileges

The class calls Impersonate upon begin(), and Revert
upon end() or destruction.

The user is the current user that is logged in.

 create mode 100644 vdagent/as_user.cpp
 create mode 100644 vdagent/as_user.h
file_xfer: create file with user privileges
vdagent: file-xfer: make user desktop the target directory

Before, the target directory was a public one (for all users),
as file creation was not done with user privileges.

Now that the file is created with user privileges, it's
better to make the user desktop the target directory.
vdagent: file_xfer: make g_key_get_string safer

By providing the size of the destination string buffer.
Makefile.am: tabify
Makefile.am: add INCLUDES to AM_CPPFLAGS

Fixes the following autoreconf (automake) warning:
Makefile.am:6: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')