Target a C99 subset, not a C89 subset

Target a C99 subset, not a C89 subset

For many years Gnulib has targeted C89 and has resisted using C99
features, as some Gnulib-using programs still wanted to target
C89.  As this no longer seems to be the case, relax the porting
requirements to allow some C99 features.  This is merely a change
to the documentation, to give other Gnulib developers a chance to
weigh in on the topic.
* doc/extern-inline.texi (extern inline):
* doc/gnulib-readme.texi (Portability guidelines):
* doc/gnulib-tool.texi (Initial import):
* doc/gnulib.texi (Header files):
Modernize to talk about C99 and C11 instead of C89 and C99.
* doc/gnulib-readme.texi (Portability guidelines):
Now a section, not merely a subsection, so that it
can be split up.  Modernize a bit.
(C language versions, C99 features assumed)
(C99 features avoided):
New sections.