doc: List the target platforms.

doc: List the target platforms.

* doc/gnulib-intro.texi (Target Platforms): New section.
* doc/gnulib.texi (Introduction): Update menu.
* README (Portability guidelines): Refer to the new section. Update
statement about oldest supported environment. Remove rationale why
<errno.h>, <string.h>, <stdlib.h> are assumed. Update example of an
unportable C89 function.
Reported by Bastien Roucariès <roucaries.bastien@gmail.com> and
Charles Wilson <cygwin@cwilson.fastmail.fm>. Feedback from Paul Eggert.