fix(sysseek): return new file position instead of success flag

[fglock]

Summary

Fixes jcpan -t File::Tail, which hung forever during its test suite.

Root cause

sysseek was aliased to seek, which returns 1 on success. Perl's sysseek is documented to return the new file position (with the special "0 but true" dualvar for position 0).

File::Tail::checkpending uses sysseek($fh, 0, SEEK_END) to detect new data at the tail of the file. With the old behavior endpos always came back as 1, so endpos - curpos == 0 and the polling loop in File::Tail::read never made progress.

Minimal repro (before fix):

open(W,">/tmp/f"); print W "hello\n"; close W;
open(RD,"<$name");
print sysseek(RD,0,2);   # printed "1", expected "6"

Fix

• Added a proper sysseek(RuntimeScalar, RuntimeList) in IOOperator that performs the seek, then returns the new position via tell, mapping position 0 to the "0 but true" string.
• Rewired OperatorHandler so sysseek dispatches to the new method instead of reusing seek.
• Updated the (int ctx, RuntimeBase...) adapter used by MiscOpcodeHandler accordingly.

Test plan

• ./jperl round-trip of sysseek with all whence values returns correct positions (including "0 but true" for position 0).
• jcpan -t File::Tail now reports Files=3, Tests=15, Result: PASS.
• make (full unit test suite) passes.

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