ITPRO-86 Lock write access to lab.conf, fixes #9 and helps #7
Read-write Lab objects now lock the lab.conf during their entire
life time, thereby guaranteeing that nobody else will use lab.conf in
this time. Read-only Lab objects will wait till another writing process
releases the lock on the lab.conf.

Locks should also work over NFS so that LML can be used on multiple
hosts. However, this is no solution for DHCP leases, only for the LML
state data.
ITPRO-86 Make extra link text customizable, closes #25
Also remove GECOS lookup for VM Overview because the lookup is rather
slow on LDAP-backed servers. If we want to show more than the username
then we should write it in LML::Lab::update_vm so that we do the
expensive calculation only once and not on each user viewing the VM
list.