Remove obsolete restriction on the range of log_rotation_size.

Remove obsolete restriction on the range of log_rotation_size.

When syslogger.c was first written, we didn't want to assume that
all platforms have 64-bit ftello.  But we've been assuming that
since v13 (cf commit 799d22461), so let's use that in syslogger.c
and allow log_rotation_size to range up to INT_MAX kilobytes.

The old code effectively limited log_rotation_size to 2GB regardless
of platform.  While nobody's complained, that doesn't seem too far
away from what might be thought reasonable these days.

I noticed this while searching for instances of "1024L" in connection
with commit 041e8b95b.  These were the last such instances.
(We still have instances of L-suffixed literals, but most of them
are associated with wait intervals for pg_usleep or similar functions.
I don't see any urgent reason to change that.)