common: Add utils.[ch] with a string_concat() function

Instead of doing malloc() and repeated strcat(), which is an O(n^2) way to concatenate multiple strings,
we define a single O(total_len) function that uses stpcpy().  This will also make the rest of the code
more legible and safer.
Bug #331 - Don't create a /tmp/root directory insecurely

When finding the user's home directory to generate a subdirectory in $HOME/.config, we would
fall back to /tmp if there were no environment variables for HOME or XDG_CONFIG_HOME.

Since libimobiledevice gets used by upower, and since upowerd runs as root, this would cause
a /tmp/root directory to be created insecurely, leaving upowerd vulnerable to a symlink attack.

Now we fall back to getpwuid_r() to find the user's home directory if it is not provided
in environment variables - this is the case when upowerd gets run via systemd, for example.
The result is that we'll end up creating /root/.config, a safe directory, since regular
users cannot create symlinks in /root.

In the future we'll need a way for libimobiledevice to find where to store its pairing data
on behalf of the console user, rather than writing it to /root.

http://libiphone.lighthouseapp.com/projects/27916-libiphone/tickets/331-insecure-tmp-directory-use
configure.ac: Simply check for asprintf and vasprintf
common: Implement stpcpy() replacement for systems lacking it
common: Fix WIN32 portability for recent userpref security fix
common: Fix malformed binary debug message dumping
userpref: correctly NULL-terminate arguments in string_concat calls
Fix memory leaks in instproxy_client_get_path_for_bundle_identifier()
installation_proxy: Fix another two memory leaks
Update NEWS with latest changes
syslog_relay: Add doc comments for syslog_relay(_start|_stop)_capture()
Update README with latest code, ticket tracker and IRC URLs
Enable unback of encrypted backups.

Adds the supplied password to the Unback request, which is sent in the main message, not in an options dictionary.
Merge upstream 'master' into osx
Fixed build settings after merge from upstream.
Added recursive operation to cmd_rm().