contrib: main.mak: Guard downloads with flock.

This is to prevent resource contention on shared file systems such as
NFS.

* contrib/src/main.mak (download) <wget>: Guard with flock.
<curl>: Likewise.
<fetch> Likewise, and properly quote shell arguments.
(download_git): Likewise.

Change-Id: I8e2139d780a34ad054cfa8e188c6cbaf94df0609