send_cmd: use (practically) infinite retries for writers

send_cmd: use (practically) infinite retries for writers

Write tools (-*index, -watch, -mda, lei) should never croak due
to the system being busy.  So make the retry infinite to benefit
users who run several parallel imports at once on a slower
system.  The previous 5s timeout was too close to failing in
my own experience using `lei import' on an old, busy machine.

For lei (inotify || EVFILT_VNODE) watches, we now retry on busy
sockets to avoid loss of FS change notifications.

On the contrary, public-facing read-only interfaces have always
been assumed to constantly be under attack.  Thus continuing to
drop requests due to a lack of kernel memory/buffers is probably
prudent.