* Fix 2 problems. First, 0 is a valid fd on Unix (easily gotten if user first
closes all fds and then starts using memfiles). Use -1 instead for an invalid
fd. Second, it is best practice to conserve open fds on Unix and file handles
on Windows. These handles are not needed unless the user wants to remap the
memory with ``mapMem`` (or a hypothetical future ``proc resize``). Adding a
new bool param ``allowRemap=false`` to ``memfiles.open`` solves this cleanly
in a "mostly" backward compatible way. This is only "mostly" because the
default ``false`` case does not keep unneeded resources allocated, but that
most sensible default means that any ``mapMem`` callers need to fix all their
open calls to have allowRemap=true, as this PR also does for tmemfiles2.nim.
* Include backwards compatibility note.
* add 'last' argument to 'find' procs in strutils
* add 'rfind' proc for looking up set[char] in strutils
* use optimised C function 'memchr' when available
* When formatting timezone, substract 1 hour from timezone when isDST
* Do not depend DST in current timezone when parsing arbitrary date
because formatted timestamps are never in DST.
* On the way, removed an unnecessary line in parsing code which could
cause bugs.
* Added DST tests
* No mapping between TimeInfo.tzname and TimeInfo.timezone
* tzname of time.h is not well-defined, may have almost arbitrary
length, and localization may differ
* Code used hardcoded "UTC" string
`initInterval` had logic to calculate and carry overflowed fields (65 seconds
turns into 5 seconds and carries 1 minute). However, we were not including that
carried value when we recalculate the carry over for the next period of time.So
if you had, for example, 3600 seconds, we carried 60 minutes into the minutes
calculation, but when we calculated how much we should carry into the hours
value we only considered what the user originally supplied for the minutes
field, and forgot to include those 60 carried minute.
So, for example, with the previous implementation this was true:
`seconds(60 * 60 * 24) == seconds(0)`
Or, as failing tests:
```nimrod
import times
assert seconds(60 * 60 * 24) != seconds(0)
assert seconds(60 * 60 * 24) == days(1)
```
