global: Use libelogind when libsystemd is not available

[tbzatek]

Elogind ships with the libelogind library that is built upon libsystemd sources. Symbol names, header file naming and most of its original functionality is preserved.

https://github.com/elogind/elogind

Closes #17

[tbzatek]

Quick and dirty patch. Nearly untested. It compiles, executes and reacts on usid commands.

$ usid version
USID_PROTOCOL=1
USID_MAJOR=0
USID_MINOR=0
USID_RELEASE=2
SID_PROTOCOL=1
SID_MAJOR=0
SID_MINOR=0
SID_RELEASE=2
$ usid active
USID_BRIDGE_STATUS=active

I'm getting lots of errors like

<command/20543/scan> scan-next phase failed.
<command/20543/scan> Failed to execute command

though they might be unrelated to this change.

[tbzatek]

# sid -f -v
^C<sid> Event loop failed.: Unknown error 1419826320.

[prajnoha]

I'm getting lots of errors like

<command/20543/scan> scan-next phase failed.
<command/20543/scan> Failed to execute command

though they might be unrelated to this change.

Yes, that's expected for now - this is because of missing modules to handle specific device types (I should probably change/add better message here so it's more clear what's wrong).