New tests covering INPUT_NEED_NL flag handling.  One exposes a bug.
Mark io-35.18b test as knownBug
Proposed fix for [2f7cbd01c3].
Fix [2f7cbd01c3]: tcl8.6.1 fails to build on FreeBSD 10.0
Test iogt-2.4 is another segfault demo for [721ec69271].
Test iocmd-23.11 demos another segfault.
Minimal patch to fix iocmd-23.11.  Might not be the best fix, but is *a* fix.
Cherry-pick [c54059aaad] from trunk: Added support for reporting TEA-like info via pkg-config. 
Add missing @TCL_LIB_FLAG@ (derived from ticket [5bcb5026ad])
Set return message in close if a flush error is reported (which may be an error from a background flush). Ticket [97069ea11a]
Fix test failure socket-2.9: "1 {not owner}" instead of "1 {couldn't open socket address already in use}" by only setting returned error message if not jet set.
Imported for bug [97069ea11a] from branch bug-97069ea11a: empty error message on stalled background flush after async socket connect fail