locks: tidy lock acquisition

Combine the check+create done in each script. (The original functions are still callable as before, but a new combined Aquire_lockfile function can be called instead, as now used).

Note, a further simplification could be done in removing the passing of the lock filename in as a parameter since every use of the functions is with ".lock". The lock functions already have a fallback to ".build/lock" though. Checking the history, the fallback used to be for a system wide lock, which was then replaced with this config-tree specific one. As long as that is not used implicitly by 3rd-party hooks then surely we are free to change the fallback to ".lock" and further remove passing in a name as a param...?

history: db5d2b0d 0aa8289a

Gbp-Dch: Short Closes: #952918