doc: document exec* = spawn+exit bug with non-Cygwin Windows platforms

Problem reported by Eli Zaretskii in:
http://lists.gnu.org/archive/html/bug-grep/2014-05/msg00118.html
* doc/posix-functions/execl.texi (execl):
* doc/posix-functions/execle.texi (execle):
* doc/posix-functions/execlp.texi (execlp):
* doc/posix-functions/execv.texi (execv):
* doc/posix-functions/execve.texi (execve):
* doc/posix-functions/execvp.texi (execvp):
Mention spawn+exit problem on non-Cygwin Windows platforms.
doc: document mbrtowc and mbrlen problem with empty input

* doc/posix-functions/mbrlen.texi (mbrlen):
* doc/posix-functions/mbrtowc.texi (mbrtowc):
Document portability problem when the input string is empty.  See:
https://sourceware.org/bugzilla/show_bug.cgi?id=16950
mbrlen, mbrtowc: fix bug with empty input

* lib/mbrtowc.c (rpl_mbrtowc) [MBRTOWC_EMPTY_INPUT_BUG]: Fix the bug.
* m4/mbrlen.m4 (gl_MBRLEN_EMPTY_INPUT): New macro.  It's not used,
so this is mainly for documentation.
* m4/mbrtowc.m4 (gl_MBRTOWC_EMPTY_INPUT): New macro.
(gl_FUNC_MBRTOWC): Use it.
* tests/test-mbrtowc.c (main): Test for the bug.
mbrtowc.m4: fix a comment typo

* m4/mbrtowc.m4 (gl_FUNC_MBRTOWC): Fix typo s/2/-2/ in
emitted documentation string.
getlogin-tests: avoid false failure under cron

* tests/test-getlogin.c (main): Avoid verifying errnos from ttyname()
since that's not what's under test.  Centos 6 was seen to return
EINVAL for ttyname() when run from cron.