hostname: improve the algorithm in hostname_pick_word()

hostname: improve the algorithm in hostname_pick_word()

Lennart suggested to use a more uniform algorithm for
the picking of the hostname words that is not biased
for long words by just (predictably) randomly going over
the offsets until we land on a word boundary. This is a
very nice suggestion so this commit implements it with
a fallback to the "old" behavior if we do not find a
word boundary within a reasonable amount of attempts.

A small python script shows that 64 iterations plus
fallback is a good number:
```
$ python3 simulate-hostname-pick.py 64
hostname-wordlist/adverbs
  words=261  p_accept=0.1119  avg_bytes/word=1/p=8.94
  max_iterations=64, n_trials=1000000
    fallback rate       :   0.051000%  (510/1_000_000)
    mean seeks per word :        8.93

hostname-wordlist/adjectives
  words=449  p_accept=0.1380  avg_bytes/word=1/p=7.24
  max_iterations=64, n_trials=1000000
    fallback rate       :   0.007500%  (75/1_000_000)
    mean seeks per word :        7.25

hostname-wordlist/nouns
  words=449  p_accept=0.1472  avg_bytes/word=1/p=6.79
  max_iterations=64, n_trials=1000000
    fallback rate       :   0.002700%  (27/1_000_000)
    mean seeks per word :        6.79
```
Combined with the fallback to the previous method if
we can't find anything within the 64 attemps this seems
to be the best tradeoff and give us very good uniformity.