Move lots of non-Serval-specific string functions from "serval.h"/dataformats.c
to "str.h"/str.c.
Add str_fromprint() function that reverses the effect of toprint(): interprets
escape sequences in the source string "\n\t\r\0\xHH\\" and replaces them with a
single char in the destination string.
Servald starts DNA helper, receives startup ACK, sends requests, receives
responses, handles malformed helper responses, echoes dnahelper stderr lines to
log, sends MDP reply packet, waits for dead helper process, all asynchronously.
Shuts down helper process during servald shutdown.
Remaining issues:
- Does not impose a timeout on helper responses.
- Only the first URI is reported by the "dna lookup" command.
SET_NONBLOCKING(), SET_BLOCKING(), WRITE_STR() are now set_nonblock(),
set_block() and write_str() respectively, all of which log an error before
returning -1. There are other useful methods: write_all() treats anything less
than all bytes written as an error; write_nonblock() treats EAGAIN and EINTR as
zero bytes written, and a combination: write_all_nonblock().
... as part of working on work on DNA helper callout facility.
(DNA replies now contain a token that can be used to match them
against DNA requests as a happy side-effect).
