[PATCH] increase fd_limit to max_client_limit automatically

Hi, with a low soft limit on file descriptors, dovecot 2.2.15 warns on startup:

Warning: fd limit (ulimit -n) is lower than required under max. load
(256 < 1000), because of default_client_limit

It could try increasing the limit first, and only report the warning if that fails. I'm attaching a patch that does just this.

Without the patch, the soft fd limit is kept at whatever it was on dovecot startup:

% pfexec plimit $(pgrep dovecot)
18737:  /opt/niksula/sbin/dovecot
   resource              current         maximum
  time(seconds)         unlimited       unlimited
  file(blocks)          unlimited       unlimited
  data(kbytes)          unlimited       unlimited
  stack(kbytes)         10240           unlimited
  coredump(blocks)      unlimited       unlimited
  nofiles(descriptors)  256             65536
  vmemory(kbytes)       unlimited       unlimited

and with patch applied, it's increased to max_client_limit:

% pfexec plimit $(pgrep dovecot)                                 
18775:  /opt/niksula/sbin/dovecot
   resource              current         maximum
  time(seconds)         unlimited       unlimited
  file(blocks)          unlimited       unlimited
  data(kbytes)          unlimited       unlimited
  stack(kbytes)         10240           unlimited
  coredump(blocks)      unlimited       unlimited
  nofiles(descriptors)  1000            65536
  vmemory(kbytes)       unlimited       unlimited

It should probably be increased to higher than that to account for fds other than client sockets, but I don't have insights into that.

-- Lauri Tirkkonen Niksula systems specialist