build: support systems without O_CLOEXEC

[steadytao]

Fixes #1100

Summary

• move the existing O_CLOEXEC fallback into the shared header
• remove the duplicated function-local fallbacks
• add a compile-only check for rsync.c without O_CLOEXEC
• update the existing authentication portability regression to exercise the shared fallback

[steadytao]

@idgn23 Could you please verify?

[steadytao]

@idgn23 Bump.

[idgn23]

Will verify within this week. Had a busy week last week.

[steadytao]

No worries. Please take as much time as you need.

[idgn23]

Looks better, but there is now a different issue:
/opt/SUNWspro/bin/cc -I. -I. -I./zlib -xO3 -m32 -xarch=sparc -DHAVE_CONFIG_H -I/opt/csw/include -c params.c -o params.o
gawk -f ./define-from-md.awk -v hfile=default-dont-compress.h ./rsync.1.md
/opt/SUNWspro/bin/cc -I. -I. -I./zlib -xO3 -m32 -xarch=sparc -DHAVE_CONFIG_H -I/opt/csw/include -c loadparm.c -o loadparm.o
/opt/SUNWspro/bin/cc -I. -I. -I./zlib -xO3 -m32 -xarch=sparc -DHAVE_CONFIG_H -I/opt/csw/include -c clientserver.c -o clientserver.o
"clientserver.c", line 1621: undefined symbol: O_DIRECTORY
cc: acomp failed for clientserver.c
gmake: *** [Makefile:76: clientserver.o] Error 2

[steadytao]

Oh. The other shared flag is broken too. Alright, give me a bit.

[steadytao]

Thank you.

[steadytao]

@idgn23 When you are next available please.