Simplify locking by using lockf(3). It is POSIX compatible and should
work over NFS.
Fix download race condition when:
1) host A creates lockfile and aquire lock to fetch from distfiles
mirror
2) host B opens the lockfile and waits for lock
3) host A gets 404 from distfiles, releases lock and deletes the
lockfile, which host A has an open file handle for
4) host B gets lock of the deleted file and downloads file
5) host A retries download and creates a new lockfile, but is not
blocked by host B, even if it should
Solve this by releaseing the lock, give the other processes a chance
to aquire it (using sleep(0)), and then only delete the lockfile if:
a) download was successful (no 404) or b) no-one else has a lock.
This reverts commit 281720ec39 (abuild-fetch: aquire a second lock
using flock(2))
fixes#10026
3da770ce35